A white-lighter is someone who insists on spiritual purity and avoidance of any less than saintly activity. This attitude comes primarily from Wiccans and other Pagans, who have been for many years called evil by condemning Christians and uninformed agnostics. White-lighters have spent quite a lot of effort on giving themselves and their religious beliefs a spotless public appearance. The term derives from constant assertions that these people derive their power strictly from "white-light energy" and not from darkness.
This term is sometimes used in a derogatory fashion toward someone who obsesses needlessly over the purity or sanctity of any act, energy, or working performed or discussed in their presence. The thought here is that Wicca has become generally accepted as a "real" religion by much of mainstream western society, and therefore no longer needs to prove itself as any more publically acceptable than the other religions which surround it; the insult also implies that the subject carries an unwarranted "better than thou" attitude toward others.