Psionics is similar to magic, in that it is the ability to do shit that the laws of physics say you shouldn't, but is considered more inherently "science fictiony" than magic, which is seen as the realm of fantasy.
The general definition of psionics is that it is a power that stems from the user's mind, and as such it tends to focus more on mental effects; telepathy, mind control, hallucinations/illusions and so forth. However, in "softer" settings, taking inspiration from the real-world belief in telekinesis (the ability to move physical objects by willpower alone), psionics can get much more flashy, until the border between it and regular magic boils down more to semantics than anything else.
Dungeons & Dragons
Psionics has a long tradition in Dungeons & Dragons, and two of its most iconic aberrations, the Aboleth and the Illithid, are defined by their connection to it (or to magic, depending on edition). However, it's long been the black sheep, to the point that Gary Gygax himself thought that it was a bad idea to include it.
Seriously, long-running consensus on /tg/ is that Dungeons & Dragons' psionics is cursed - that these books are not RPG supplements, but grimoires of a sadistic cult, playing with your emotions, and to have any D&D psionics book present in the room will ensure you always roll poorly. Another common opinion is that psions in D&D are overpowered. This stems time and time again from game designers trying to make psychic powers something different than spellcasting, and fucking up the game design doing it. But despite all the bitching, Third Edition psionics is still ten times easier to understand than how wizards worked, and not as powerful as a cleric abusing the shit out of uses turn/rebuke undead attempts as fuel.
Psionics were the focus of one of the most significant changes between 3E and 3.5. In contrast to the previous edition, 3.5e psionics are essentially just a casting system that uses Power Points instead of Vancian Casting. Unlike previous attempts, 3.5 Psionics are well received by people who actually read the rules, but are often wrongly accused of being overpowered by people who missed the (admittedly not very prominent) rule that you can't use more PP than your level on a power.
Ironically, 4th edition D&D actually had the easiest, simplest, most well-integrated psionics system seen in D&D to date. In 4e, psions are still mechanically different, but not to game-tangling extents: psionic classes don't get Encounter powers. Instead, they get a much larger array of at-will powers and a level-determined amount of "power points". Psionic at-wills feature a mechanic called "Augmentation", where a PC can spend some of its power points to buff up the effect. Psionic powers also place much heavier emphasis on the Psychic and Force damage types, making them much harder to resist for most critters.
Traditionally, D&D psionics was divided into six psionic schools, in a repeat of the magic system:
- Clairsentience covered psionic powers that related to expanded sensory perception. A Psion specializing in this discipline is known as a Seer.
- Psychokinesis covered psionic powers that physically affected the world. A Psion specializing in this discipline is known as a Kineticist.
- Psychometabolism covered psionic powers that modified and mutated the body in various ways. A Psion specializing in this discipline is known as a Egoist.
- Psychoportation covered psionic powers relating to manipulating space. A Psion specializing in this discipline is known as a Nomad.
- Telepathy covered psionic powers relating to direct mental communication. A Psion specializing in this discipline is known as a Telepath.
- Metapsionics covered... well, basically a grab-bag of various powers that didn't mesh into any of the others. Later versions changed this to Metacreativity, focused on making and changing stuff with ecotoplasm. A Psion specializing in this discipline is known as a Shaper.
Dungeons & Dragons initially introduced psionics as just "inherent powers", special abilities you could get if you had really, really lucky rolls. AD&D 2nd edition introduced the first ever psionic class, the Psionicist, which was all about growing in psionic powers. 3rd edition renamed the Psionicist to just the Psion and introduced further psionic classes, many of whom were just specific flavors of psionicist or "psychic versions" of existing classes, like the Soulknife, Erudite, Wilder and Psychic Warrior. 4th edition divided psionics up between three classes - the Psion, Ardent and Battlemind - although the Monk was also considered to use the "Psychic Power Source".
D20 Modern includes Psionics in its core rulebook, albeit as a variant. This is expanded on in Urban Arcana. This is the first time Psionics appeared under Open Gaming License, which may be why the 3.5 incarnation was OGL, since it's clearly derived from this sytem. They're a cross between 3E and 3.5 psionics, using many of fixes of 3.5 psionics but still requiring various attributes for each discipline. Augmenting powers is completely absent from this incarnation and, like spells, powers scale naturally with manifester level.
- Telepath has a bunch of mental powers and gets extra PP from its Charisma and is, oddly, the only one of the four to get bonus power points for high attribute score (which may be an oversight). Until the final D20 Modern book, Dark Matter, they had virtually no options for offensive powers. Their power list is the most focused attribute wise. With only five exceptions based on wisdom (one a cantrip, and one utter crap) all powers with saving throws are based on Charisma and others are based on Dexterity or Constitution.
- Battlemind is a combination of Psychic Warrior and Soulknife. Since magic items are rare and not expected in D20 Modern, full BAB is rare, and hiding your weapons is more of a concern, mindblades are actually quite decent. Their power list most focuses on buffs, but is very scattered in attribute dependencies.
- Psionic Agent was introduced in Urban Arcana and focuses on movement powers. They other depend on shooting stuff. Mostly dexterity based, but wants wisdom and intelligence as well for some important powers.
- Mesmerist was introduced in D20 Past. They only get a fraction of the normal psionics progression and no other class abilities. Supposedly for low psionics games, this class is Truenamer tier broken since it doesn't actually say what powers it can take as powers known (the example character pulls from the telepath list, but WotC example characters are wrong, often blatantly, more than they are right.).
Psionics are also referenced in D20 Future and D20 Apocalypse, but only as special abilities of aliens and mutants. Unlike casters, there is no Psionic Prestige Class to continue advancing after hitting level 10. This role is instead filled by the ability to multiclass and stack power point progression, but it's not as nice a deal since you don't get as many extra castings (four levels of Archmage doubles your casting slots. Four levels of an extra psionic class doesn't even give you an extra casting of a 5th level power.) and don't increase your caster level. Despite this problem and the all attributes thing they're still casters in a system where, despite the many nerfs they got, casters are still king.
Pathfinder, of course, has its own "psionic system", which it tries to differentiate by using real-world beliefs in psychics and occultism. The end result for all of them but Kineticist was just another set of Vancian magic spells with some components swapped around and using under-supported, proprietary spell lists. It was introduced to the world in the Occult Adventures sourcebook, which was followed by the far-shorter Occult Bestiary, for more psionic-focused enemies.
- The Kineticist draws upon an innate ability to manipulate one or more elemental forces and energies through the power of their will.
- The Medium communicates with the spirits of the dead and allows them to possess their body in order to gain temporary powers drawn from the spirit.
- The Mesmerist is a specialist psychic who focuses on mind-controlling powers powers exclusively, essentially a psionics-fueled enchanter & illusionist.
- The Occultist seeks out relics imbued with strong psionic resonance and uses these as a medium to draw upon and channel psionic energy; without these totem-implements, they're all but powerless.
- The Psychic is essentially the Pathfinder version of the Psion: a master of various psychic powers that derives their strength from a fundamental discipline, functioning somewhere between a psionic sorcerer and a specialist wizard.
- The Spiritualist is similar to the Medium, in that its powers stem from a psychic connection to the dead. However, the Spiritualist has a singular bonded spirit, a Phantom, and this entity can carry out most of the fighting for them, using the Spiritualist as a battery to enhance its abilities.
|The Classes of Pathfinder|
|Core Classes:|| Barbarian - Bard - Cleric - Druid - Fighter - Monk |
Paladin - Ranger - Rogue - Sorcerer - Wizard
| Alchemist - Antipaladin - Cavalier |
Inquisitor - Oracle - Summoner - Witch
| Arcanist - Bloodrager - Brawler - Hunter - Investigator |
Shaman - Skald - Slayer - Swashbuckler - Warpriest
| Kineticist - Medium - Mesmerist |
Occultist - Psychic - Spiritualist
|Ultimate X:||Gunslinger - Magus - Ninja - Samurai - Shifter - Vigilante|
However, for those who want the more classic 3.X Experience, 3pp producer Dreamscarred Press also ported forward the Psionics system, with a few new bits. The biggest changes here are the total destruction any remnants of non-transparency between psionics and normal magic beyond their separate knowledge skills. This type of psioncs is technically canon in Golarion, as there's a handful of mentions from back when Pathfinder was third party supplements for 3.5.
- The Psion is still here and the primary casty-type with Wizard-tier Discipline specializations. Generalist option has been added
- The Wilder is also a caster, but a much more offensive sort who has to balance how they use their surges and the risk of Psychic Enervation.
- The Psychic Warrior is a mid-tier Psionics character, who blends together Fighter-Tier feat availability with psychic focuses based on how you want them to work.
- The Soulknife is kinda Psionic in name only. The blade itself is their sole power, and then they get Blade Skills (talents) that give their blades new uses and perks. Thankfully they have an archetype giving them limited Psionic powers.
- The Dread is a class based on being big and scary and touching things in order to inflict all sorts of conditions on them.
- The Marksman is a low-tier Psionic class that focuses more on shooting things like a Ranger.
- The Cryptic is the Psionic Rogue, only with strange ways to mess with people by screwing with reality in small ways.
- The Aegis is kind of like the opposite number to the Soulknife: rather than making weapons, these guys make suits of psionic armor (available in either barely-there skin, average armor, or heavy behemoth armor) and selecting a set of mods to give their armor special functions.
- The Tactician is a psionic mastermind, a combat leader with better powers. They focus on gathering allies into a collective consciousness and spreading out bonuses and benefits to the members when necessary.
- The Vitalist is the answer to anyone complaining that Psionics has shit healing. Like the Tactician, they work based on a collective and spread out any heals among the party. Unlike most healer classes, they're actually quite acceptable at things other than healing. Psionics makes it really easy to get powers from another list and one options built into the class focuses on vampiric healing.
Also introduced was the "seventh path", a new discipline to address the lack of a necromancy counterpart.
- Athanatism covers psionic effects that deal with spirits and ghosts, essentially psionic necromancy. A psion specializing in athanatism is known as a Conduit.
Warhammer 40,000 is the premier /tg/ example of the "psionics is just space magic!" design. Psionics in the 40kverse derives from the ability to mentally tap into the Warp, a parallel universe of pure, unbridled emotional energy, where thought and matter are interchangeable. As all minds are inherently linked to this dimension, psykers just have the ability to exploit that link, psionics consists of either using the Warp as a way to directly connect to and influence the minds of others, or drawing Warp energy into the material universe and using it to temporarily rewrite the laws of reality.
The similarities to magic are actually admitted to and even lampshded; using magical trappings can help focus the mind and make it easier to work psionics, whilst "sorcery" is a real thing -- it amounts to psionics and bartering with (or enslaving, for the stronger casters) daemons and using them as batteries of psionic energy.
In practice, actually using psionics in 40k is generally a bad idea. Any potential upside of using a psyker's powers has to be weighed against the very real possibility of making things worse. Psykers are regarded less as beings and more as unexploded ordinance by both humans and orks, while the eldar are trained from infancy to wall off their powers.
The Force in Star Wars is just a fancy name for psionics. Ironically, although treated like magic, it functions more like classic psionics; Force Users predominantly produce mental effects (the famous "Jedi Mind Trick" is basically a psychic Charm Person spell) and telekinesis, although there are more exotic powers as well - most of these tend to be Dark Side, though.
Ironically, despite the vast amount of weird space shit that pops up in Star Trek, psionics isn't very common. Vulcans have the ability to telepathically probe and manipulate minds through a meditative trance that requires physical contact, the infamous "Vulcan Mind Meld", whilst Betazoids are characterized as a telepathic species, although half and occasionally even quarter Betazoids are empathic (emotion sensing). Other noteworthy telepathic species include the Talosians and the Ocampa.