The Psion is a Dungeons & Dragons character class that uses psionics. That's really all there is to it, which you can already tell is a remark on just what kind of relationship D&D has with psychic powers in PC hands.
In 1st edition, psions were normal characters that rolled 99-100 on d100 during chargen, and gained extra spellcasting that didn't require spellbooks nor devotion to a faith. They had their own psionic-only battles that were invisible to non-psionics, and required their own combat matrix (complete with only one always optimal attack and one always optimal defense) on the DM's screen so you could play rock-paper-scissors-lizard-spock with the five attacks and five defenses. Only one attack could actually affect non-psionics, and it was less effective than casing Feeblemind or just punching someone in the jaw. Everyone else would just stand around and watch 10 rounds of an invisible, silent fight between people standing perfectly still for each round of actual combat. Whoop-de-doo.
Can you tell Gygax didn't even want to include it yet?
In 2nd edition, the Complete Book of Psionics introduced the Psionicist class, which was dedicated to having psychic powers. It was the precursor to the Psion in every way that really matters, but it still relied on the same godawful psychic combat mechanics as 1e.
D&D 3.0 had horrific mechanics: the ultimate M.A.D., where you needed a high stat for every subcategory of psionics to have a decent power. There were no supplements for 3.0 psionics, and we're okay with this.
In 3.5e, psionics were simpler: Psions were accountant spellcasters who kept track of a mana pool. Psionic powers didn't level up (well, most of them), instead the psion would spend mana ("power points") to increase a power's effect. Other players raged about this being h4x because a psion could turn themselves into glass cannons and screw a BBEG if they really wanted to. Most of the complaints about psionics came from people who didn't actually read the rules entirely (Few noticed the the rule saying you can only spend your manifester level in PP at once stopping you from dumping your entire pool into something) from pro spellcasters, who want to be the only ones with an "I win" button.
3.5e psionics only had one good main class: the Psion, or psychic sorcerer. Psychic Warrior was just a better fighter (which isn't saying much). The wilder was a psionic warlock, the Soulknife was a shitty knock-off of monk WITH LIGHTSABERS11!!ELEVEN11!!! just like that hot Asian chick in X-men comics. The other classes described in the Complete Psionic book aren't worth mentioning, except the Erudite, or psychic wizard, which was as good as CoDzilla or a Wizard.
The 4e Psion was the Psychic Controller class, making it analoguous to the Wizard or Invoker. By the time 4e was cancelled, it could be divided into three subclasses; one focusing on telepathy to control peoples' minds, the second focusing on telekinesis to fling people around the battlefield like toys, and the third on creating psionic constructs to do the dirty work for them.
The Psion has yet to turn up officially in 5th edition, although the newcomer Unearthed Arcana class the Mystic has basically taken not only its place, but the role of every single psionic class of editions past. A surprisingly official-looking homebrew expasion of that UA has been made by fans, which comes with additions such as new "magic" items and new psionic monsters. All in all you're actually better off going with this than the actual UA: https://drive.google.com/file/d/0B2bXgsHg7PtJNXdRdjdaUDhRM2M/view?fbclid=IwAR29j5mT2gBK87ZWAbDoelfk-k1YQQuZMgDwvUXYbYQjW6FU1gZu0U10eGU
The Pathfinder third party supplement Ultimate Psionics contains the Psion class, which works more or less the same as it does in 3.5e. Of the official PF psychic classes, the Psychic probably closest fits the general archetype of Psion as the "undiluted psionic master" class.
Mechanically, the Psychic functions akin to the Sorcerer, with most of its power stemming from the various Disciplines (sources of psionic energy) that it chooses.
|Dungeons & Dragons 4th Edition Classes|
|Player's Handbook 1:||Cleric - Fighter - Paladin - Ranger - Rogue - Warlock - Warlord - Wizard|
|Player's Handbook 2:||Avenger - Barbarian - Bard - Druid - Invoker - Shaman - Sorcerer - Warden|
|Player's Handbook 3:||Ardent - Battlemind - Monk - Psion - Runepriest - Seeker|
|Heroes of X:|| Blackguard - Bladesinger - Binder - Cavalier - Elementalist - Hexblade - Hunter|
Mage - Knight - Protector - Scout - Sentinel - Skald - Slayer - Sha'ir - Thief
Vampire - Warpriest - Witch
|Settings Book:||Artificer - Swordmage|
|Others:||Paragon Path - Epic Destiny|
|Dungeons & Dragons 5th Edition Classes|
| Barbarian - Bard - Cleric - Druid - Fighter - Monk |
Paladin - Ranger - Rogue - Sorcerer - Warlock - Wizard
|Artificer - Mystic|
|The Classes of Pathfinder|
|Core Classes:|| Barbarian - Bard - Cleric - Druid - Fighter - Monk |
Paladin - Ranger - Rogue - Sorcerer - Wizard
| Arcanist - Bloodrager - Brawler - Hunter - Investigator |
Shaman - Skald - Slayer - Swashbuckler - Warpriest
| Alchemist - Antipaladin - Cavalier |
Inquisitor - Oracle - Summoner - Witch
| Kineticist - Medium - Mesmerist |
Occultist - Psychic - Spiritualist
|Ultimate X:||Gunslinger - Magus - Ninja - Samurai - Shifter - Vigilante|
| Aegis - Cryptic - Dread - Marksman - Psion |
Psychic Warrior - Soulknife - Tactician - Vitalist - Wilder
|Path of War:||Harbinger - Mystic - Stalker - Warder - Warlord - Zealot|