A Medium is a character who possesses the ability to talk to the spirits of the dead - not a necromancer in the /tg/ sense, but one in its original sense; a diviner who works their arts through communing with the dead. They may be particularly adept at fighting undead, especially ghosts, but more often than not are completely incapable of combat; they serve as flavor or advisory NPCs more than anything.
Mediums and spiritualism were hugely popular between the late 1800s and early 1900s.
The first version of a Medium to appear in Pathfinder was a new Mystery for Oracles introduced in part 5 of the Reign of Winter adventure path, "Rasputin Must Die". Inspired by the spiritualists and mediums who were prolific in Russia and the West during the time of the Great War, the "Occult" mystery grants the Oracle access to an array of spells and abilities reflecting the ghost-calling necromancer-psychic that mediums proclaimed to be.
This was followed up in the Occult Adventures sourcebook, where the Medium was introduced as its own distinctive class. The class suffers from massive MAD, as it's primary ability is to LITERALLY MIMIC OTHER CLASSES. Mediums can also talk with spirits, but unless you play in a decidedly horror/occult campaign, those abilities will mean jack shit. They get proficiency with Simple Weapons and Medium (heh) Armor. Mostly. Mediums can spirit surge, which is essentially a mythic surge you can only spend on whatever your possessing spirit likes. The medium also has an 'Influence meter' which shows how much control a spirit currently possessing him has. At 3/5 influence, your spirit starts messing with you, giving spirit specific penalties in addition to a -2 on initiative. You can also take a taboo, which is essentially taking on some other penalties (for some reason different than influence penalties) to be able to spirit surge more. Let's move on to the best part of the Medium: Spirits
There are six normal spirits, Archmage, Champion, Guardian, Heirophant, Marshal, and Trickster (which many might realize are just the mythic paths), and a large amount of 'Legendary' spirits which are spirits that must be asked very politely to possess you and are largely fucking useless (oh boy 2 points of non-stacking bleed damage per attack! This was definitely worth slaughtering peasants for!).
In general, the class is seen as the absolute weakest of the already weak lot of the Occult Adventure classes, being terribly mediocre-to-bad at everything it can conceivably do, unable to do most of it at any given time, and arguably suffering the very worst from the sourcebook's absolute fetish for undercooked penalty mechanics in the form of "influence," which piles up quick because it's required to make half the spirits even mediocre and hard to get rid of during the day. If you dig up the original playtest for the, it indicates that they were supposed to function more like a Binder, a well loved and balanced class from 3.5, but Paizo was too lazy to create more than a handful of spirits and changed them to what they are now.
|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|