Psion

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

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.

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?

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. At least in terms of how the engine worked. While clunky and cumbersome to navigate, with the right amount of thought it was possible to use it effectively, you just had to be really REALLY minmaxy about it. A Psion was fully capable of a lot of the other main casting classes abilities like; teleportation, summoning extra-planar creatures, banishing people to pocket dimensions etc. The only thing a Psionicist really couldn't do with their powers, ironically enough, was large amounts of damage. All your basic attack powers did only 1 die of damage regardless of how much you augmented them. Buuut this was a moot point due to one power you could easily obtain early on; Disintegrate. Yep, a save-or-die power in 2nd edition. You could obtain this at level 3 if you played your cards right. Sure it would cost most if not all of your points to use it, but that was a small price to pay for an instadeath ability at a time when nobody had saves worth a damn.

However, like a miracle from on above, the Psionics rules were altered in the "Skills and Powers" supplement, which would later be bundled in with the Dark Sun revised box set. Although similar in function to before, the use of powers was streamlined to be really easy and quick to play. What changed? Well, for starters instead of the weird "power score" thing, you now had a target AC for each power, as well as a special THACO used for psionic powers that thankfully scaled off your level. This made using powers far easier than ever before. But the big change it brought on was the change to psionic combat, as thanks to the Mental AC and THACO scores, it just became straight up combat which thankfully didn't drag the battle out. Psionic attack modes and defenses were changed from powers to simple proficiencies that the Psionicist got for free. There was still a little bit of rock-paper-scissors with the different defenses and powers, but it still didn't eat up much time. Once you opened up their mind (drained them of PSPs)the attack modes became powers that did serious damage or slapped the target with an unsavable debuff. Oh and the best part? Your Psionic attack modes were more effective against targets without Psionic ability.

Third edition

Yeah we know why you really want to play a Soulknife

D&D 3.0's first and only psionics splatbook was the Psionics Handbook, which was an absolute clusterfuck. It made the mistake of simultaneously introducing a psionics system AND bringing back a psionic combat system eerily similar to the one from First Edition. This psionic combat system in turn required all six of a character's ability scores to be good, making it the epitome of MAD, and it required more math than FATAL and the Apollo missions combined. The psionics system was a bit simpler. Psionic characters were accountant spellcasters who kept track of a mana pool. Psionic powers didn't level up (well, most of them), instead the character would spend mana ("power points") to increase a power's effect. Other players raged about this being h4x because psionic characters 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) or from pro spellcasters, who want to be the only ones with an "I win" button. 3.0e psionics had one godlike main class: the Psion, or psychic wizard. The only other class was the Psychic Warrior, a hybrid of psion and fighter.

When 3.5e launched, one of the first things it did was replaced the Psionics Handbook with a much better book, the Expanded Psionics Handbook. It was better because it took the entire bullshit "psionic combat" system, flushed it down the toilet, and pretended it never happened, leaving only the psionics system. It also had some other, much less important contributions. For example, it introduced a new class: the wilder, which was basically just a psion that relied on charisma rather than intelligence, making it the sorcerer to the psion's wizard while also being a bit of barbarian with the ability to get pissed for more power.

Later on, 3.5e added Complete Psionic, which brought 3 new psion-like classes: the Ardent, Divine Mind, and Lurk.

Fourth Edition

The 4e Psion was the Psionic 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.

In contrast to the rest of the AEDU System, Psions (as well as Ardents and Battleminds) lacked any class-native Encounter attack powers. Instead, they gained a third At-Will power as they leveled up (with later levels giving more At-Wills to trade off with like how a normal class would trade off Encounter attacks) and had a smaller pool of power points to augment their at-wills. The alarm of the small pool of points was offset by the fact that they refill every short rest, which made their augmented at-wills effectively encounters.

Fifth Edition

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

Mike Mearls personally would rather just use a new Arcane Tradition for the Wizard rather than bring back the psion in its own right. The resultant School of Psionics? Well, it looks like this:

ESP: At 2nd level, you gain the ability to contact other creatures' minds. You can cast detect thoughts using this ability. When you cast that spell, whether with this ability or from some other ability, it has the following modifications:

• The spell does not require concentration.
• The benefits that require you to use an action instead require the use of a bonus action.
• If you use the option to probe deeper and the target fails its Wisdom saving throw, it is vulnerable to psychic damage you inflict until the end of your next turn.

Once you cast detect thoughts using this ability, you can't use it again until you complete a short or long rest.

Telepath: Starting at 2nd level, you gain telepathy with a range of 30 feet. Additionally, when you select spells, you can choose spells from the psionic spell list, even if they do not appear on your class list.

Telekinetic Mind: Starting at 6th level, you gain the ability to transform your thoughts into tangible force. You can cast telekinesis using this ability. When you cast that spell, whether with this ability or from some other ability, it has the following modifications:

• The spell does not require concentration.
• The benefits that require you to use an action instead require the use of a bonus action.
• The spell's duration is 1 minute, and you cannot choose a new target after selecting your initial one. This restriction no longer applies once you reach 9th level.

Once you cast telekinesis using this ability, you can't use it again until you complete a short or long rest.

Superior ESP: At 10th level, your proficiency with the detect thoughts spell grows. You gain the following additional benefits when you cast the spell:

• The spell now targets a number of creatures equal to your Intelligence modifier (minimum 1).
• If you use the option to probe deeper and the target fails its Wisdom saving throw, you can choose to charm it instead of making it vulnerable to psychic damage. It makes another Wisdom saving throw at the end of its turn, ending this effect with a successful save or if it takes damage from you or your allies.

Improved Telekinesis: At 14th level, you master the full potential of your mind to project physical force into the world. When you cast telekinesis and each time you select a new target for the spell, you can instead choose a number of targets up to your Intelligence bonus (minimum 1).

Pathfinder

Had relesed the Occult Adventures which had Psionic relatied classes like Kineticist and Psychic, but were merely spellcasters that used mental spell component in addtion to other complex ablitys.

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.

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

The Third Party Classes of Pathfinder
Psionics:	Psion - Psychic Warrior - Soulknife - Wilder Aegis - Cryptic - Dread - Marksman - Tactician - Vitalist
Path of War:	Stalker - Warder - Warlord - Harbinger - Mystic - Zealot
Akashic Mysteries:	Daevic - Eclipse - Guru - Nexus - Radiant - Rajah - Stormbound - Vizier - Zodiac
Spheres of Power:	Armorist - Elementalist - Eliciter - Fey Adept - Hedgewitch - Incanter Mageknight - Shifter - Soul Weaver - Symbiat - Thaumaturge - Wraith
Spheres of Might:	Armiger - Blacksmith - Commander - Conscript Savant - Scholar - Sentinel - Striker - Technician
Spheres of Guile:	Agent - Courser - Envoy - Mastermind - Professional
Champions of the Spheres:	Prodigy - Sage - Troubadour - Dragoon - Mountebank - Necros - Reaper Warden - Crimson Dancer
Pact Magic:	Pactmaker (formerly known as Medium)