Demi-humans - Refers to anthropomorphic, generally non-hostile (towards man) creatures that may be played as characters: elves, dwarves, halflings, etc.
Humanoids - Refers to anthropomorphic, generally hostile creatures: orcs, goblins, hobgoblins, kobolds, etc.
Milieu - An unique game setting embodying the numerous possible variations in its creation, i.e. the "world" in which the adventures take place.
Trick - Any device or machination which is more likely to be solved by wits rather than force. Tricks do not necessarily involve physical harm to the characters, examples are rooms which rotate or descend to confuse mappers, statues which perform random actions, slanting passageways which take the party unknowingly to a deeper level, etc.
Undead - A class of malevolent, soulless monsters which are neither truly dead nor alive, including skeletons, vampires, ghosts, zombies, ghouls, et al.