The Expert was one of several NPC Classes introduced in Dungeons & Dragons 3rd edition.
An expert is an NPC usually very skilled in a profession or craft.
A typical expert NPC could be a very skilled professional or an elite craftsperson.
Notably, they pick any ten skills at character creation to be their class skills, meaning they can actually be min-maxed somewhat-effectively compared to their non-spellcasting NPC class brethren. In particular, Iaijutsu Focus can give them a solid damage option, Use Magic Device turns them into a knockoff caster, Tumble grants excellent mobility, Autohypnosis can make them immune to fear and poison, the social trifecta of Bluff, Diplomacy, and Intimidate lets them handle all face duties, and that still leaves two slots for things like Sense Motive, Spot/Listen, Handle Animal, Stealth, or Survival. There are also some silly builds that revolve around skill checks rather than other mechanics.
Perhaps because of this, the class was, ironically, power-creeped by the Factotum and Savant classes, who automatically get all skills as class skills without having to pick, and get actual class features.