Alignment

alignment, n., D&Dism: 1. a description of a character's general moral, ethical, and social tendencies, usu. quantified as a stat; designed to enforce consistent behavior in player characters. Ex: You can't help the injured elf-maiden -- your alignment is Chaotic Evil!
2. In some game systems, such as D&D, a character's place in a cosmological/spiritual/magical continuum based on a struggle between good and evil, chaos and law, the supernatural and the mundane. In this case, the character's alignment stat has specific consequences in the resolution of magical and clerical effects.
Ex: The blast only affects Evil creatures, so Sir Goodalot is unharmed. [GJ1]
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Addendum: The use of the term "alignment" originated in D&D. In recent years, authors of RPGs have turned away from simplistic systems like alignment stats to describe characters behavioral tendencies, instead providing personality archetypes, or basing games around membership in social groups with extensive descriptions of acceptable behavior. [EDE]
Addendum #2: This same idea is used in the "paths" in Vampire: The Masquerade. Many of the disadvantages in the GURPS system (e.g. Sense of Duty, Code of Honor, Pacifism, Vow, etc.) can be taken to create a more custom-made set of personal guidelines that an individual character lives by. [MW]
Addendum #3: Even D&D itself has taken a bender from alignment. In 4th Ed, all of the alignment-specific spells have been dropped or altered, and the old 9-path system has been pared down.