Psion

Pretend this psionic fight is an animated *.gif; it would look just the same

Psion is the D&D way of saying "psychic". It is a common opinion that psions in D&D are overpowered. This stems from time and time again of game designers trying to make psychic powers something different than spellcasting, and fucking up the game design doing it.

It is believed that Dungeons & Dragons' psionics is cursed. That these books are not RPG supplements, but grimoires of a sadistic cult, playing with your emotions. To have any D&D psionics book present in the room will ensure you always roll poorly.

AD&D

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?

Yeah we know why you really want to play a Soulknife

Third edition

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.

Fourth Edition

In 4e, psionics had their simplest rules ever; though they were still fairly unique, they didn't diverge too greatly from the template of at-will, encounter and daily powers that all other classes shared. Psionic classes were unique in that they had no encounter class powers, instead gaining new at-wills when they would gain encounter powers; however, they had a pseudo-mana system of "psionic augmentation points" (or "power points, again) that could be spent to bump up the effects of at-wills, depending on how much was spent; these spent points recovered after short rests, furthering the encounter-power similarity. There were four psionic classes in the tank/heals/dps routine: Ardents were your empaths and psychic healer leaders, Battleminds were your front-line Defenders with iron-hard skin and superspeed, Monks would wire-fu around the battlefield to be high damage Strikers, and Psions proper would be Controllers using Force-push, mass telepathy and conjuring.

Fifth Edition

Though psionics have been given a test-release as Unearthed Arcana, it seems the psion of editions past will be replaced by a new singular class called the Mystic, which features previous psion renditions as class branches or archetypes. Dumping MP into abilities to boost their effectiveness still applies, as well as abilities that require MP expenditure to even go into effect. Now, however, they feature a loadout of "disciplines"; choosing a discipline to focus on generally grants the psion a passive ability and a tiny handful of at-will abilities. Each discipline is focused around a Mystic's individual archetypes (ie a discipline is geared towards Order of the Immortal/Battlemind), though despite the focus any archetype can use them; staying in-archetype grants additional benefits alongside the ability-set, however.

Also features about ten psychic cantrips, with the Psion/Order of the Awakened getting two cantrips of the traditional-psychic stuff (mind bolts, etc) as freebies along with their known-cantrip selection. One of them is even that goofy RPG game thing where you stow away your weapon and it just vanishes into your body, except in D&D.

Pathfinder

The Pathfinder third party supplement Ultimate Psionics contains the Psion class, which works more or less the same as it does in 3.5e.

The Classes of Pathfinder 1st Edition
Core Classes:	Barbarian - Bard - Cleric - Druid - Fighter - Monk Paladin - Ranger - Rogue - Sorcerer - Wizard
Advanced Player's Guide:	Alchemist - Antipaladin - Cavalier Inquisitor - Oracle - Summoner - Witch
Advanced Class Guide:	Arcanist - Bloodrager - Brawler - Hunter - Investigator Shaman - Skald - Slayer - Swashbuckler - Warpriest
Occult Adventures:	Kineticist - Medium - Mesmerist Occultist - Psychic - Spiritualist
Ultimate X:	Gunslinger - Magus - Ninja - Samurai - Shifter - Vigilante