A Tiefling is a mythic or fictional creature in TSR's Planescape setting for AD&D. Tieflings are said to be the offspring or (more usually) descendents of demons, although this is seldom if ever stated explicitly. Their goodly counterparts are aasimar.

Cambions and alu-fiends/alu-demons are sometimes considered to be kinds of tiefling, as are fey'ri.

This was one of my very first nodes. I used 'demon' to indicate 'fiend' for the good of non-RPG-playing noders. Tief is the German for deep, and the designers state that this is the main reason for the name, although the similarity to 'Teufelchen', an imp, is useful.

Addendum: Tieflings and aasimar make perfectly good PC races in Third Edition, provided that it is remembered that their base abilities are as good as those of most other races with one character level. See Chapter Two of the Dungeon Master's Guide for full details.

Further Addendum: The newest edition of the Forgotten Realms Campaign Setting has a system in it for creating Tieflings, Aasimar and Genasi as Player Characters.