Wu Jen
Wu Jen are an Arcane Spellcasting class present on the more Chinese flavors of Dungeons & Dragons. They combine the traditional Wizard spellcasting abilities with Chinese-flavored Elementalism (Earth, Water, Fire, Wood, Metal) and often have Monk-esque Ki Powers, but also have a lot of weird taboos that they must obey or else they lose access to their magic, kind of like Paladins, or pre-4th edition Druids, Barbarians and Monks.
Advanced Dungeons & Dragons
The Wu Jen appeared in AD&D in the Oriental Adventures by Gary Gygax. They were identical to Magic Users in the Players Handbook with the exception of Ki Powers that were gained as they advanced in level and a Taboo that must be selected at 1st level. Violating this taboo resulted "in the loss of spells, illness, or other evil events" (Oriental Adventures, pg 26). Most taboos were related to issues of personal hygiene such as bathing or the cutting of hair.
D&D 3rd Edition
As in AD&D, the Wu Jen first appeared in 3.x in the Oriental Adventures D20 sourcebook, before getting reposted with maybe some cleaning up in the Complete Arcane book, alongside the Warlock and the Warmage.
The main tweak was the added focus on Wu Xing inspired elemental magic; Wu Jen had a very specific list of spells they could use, dividing them up into Universal, Earth, Fire, Metal, Water and Wood categories. This was important because at 6th level they got to pick one of the five elements for "Elemental Mastery", which meant they had +2 caster level with spells from that category (as well as from the Universal category).
Beyond that, their big thing was the ability to pick one spell every 3 levels and apply a "Spell Secret", permanently applying the effects of one specific metamagic trait (Enlarge, Extend, Still or Silent) to that spell whenever they cast it.
They still had to obey their taboos, but violating one just cut off their ability to cast spells for the rest of the day. Which was good, because they picked up one taboo at 1st level and then a new one each time they gained a Spell Secret. The silliness of these taboos was lampshaded in the book:
- Can't bathe
- Can't cut hair
- Can't eat meat
- Can't touch a dead body
- Can't drink alcohol
- Can't wear one or more specific colors
- Can't light a fire
- Can't sit facing a certain direction
- Must make a daily offering of some kind (food, flowers or incense most commonly) to spirit patrons
Pathfinder
There's no specific Wu Jen class in Pathfinder, but one could probably pull it off by taking one of the five Wu Xing-appropriate Elementalist archetypes (Earth, Fire, Water, Metal, Wood) and giving it some more Chinese flavor.
D&D 5th Edition
The Wu Jen reappeared in 5th edition as one of the six subclasses presented for the full Mystic class, which appeared on March 13 2017. Like most of its fellows, the 5e Wu Jen is ultimately based on one of the original AD&D schools of psionics - specifically, the Psychokinesis school, using psionic magic to manipulate the various elements. This is represented both by its list of associated Disciplines and by its class features, with the former directly manipulating various elements and the latter boosting its ability to interact with the elements both offensively and defensively.
At first level, it gains 2 additional Disciplines taken from the Wu Jen List - an array consisting of Mastery of Air, Fire, Force, Ice, Light & Darkness, Water, Weather, and Wood & Earth - as well as free proficiency in any two skills from a list consisting of Animal Handling, Arcana, History, Insight, Medicine, Nature, Perception, Religion, or Survival.
At 3rd level, it gains the Elemental Attunement feature. Essentially, if you haven't maxed out your psi limit, you can spend an extra psi point to ignore damage resistance when making an elemental psionic attack.
At 6th level, in a mythology gag to its origins as a Wizard variant, its Arcane Dabbler feature lets it take up to three 1st, 2nd or 3rd level wizard spells. It can fuel these by burning psi points to equate to spell slots; 2 points for a 1st level slot, 3 points for a 2nd level slot, 5 points for a 3rd level slot, 6 points for a 4th level slot, and maxing out at 7 points for a 5th level slot.
Finally, it finishes off its unique class features with Elemental Mastery, where it can boost a damage resistance it has to damage immunity for a turn by spending 2 psi points in reaction to taking damage of the appropriate type.