"Crusader" build tends to refer to a Cleric with Crusader, Sacred Fire, Nivaria's Barrier, amd Holy Shield... which are all the Cleric skills that give bonus to undead / death resist. (Not including advanced skills). In this particular game, "Crusader" tends to be used to refer to killing undead.
Noun names for builds get a bit complicated as the game allows a lot of flexibility in builds.