The single most important trait in the role-playing game Call of Cthulhu. It measures your character's mental stability, the higher the better and is a product of your character's POW. Every time you experience something traumatic, terrifying, or learn about the true nature of the universe your sanity score goes down. Read those Pnakotic Manuscripts you found during you last adventure and it goes down. Have your brain removed and stored in a jar by the foul Funghi from Yuggoth and it goes down. See Cthulhu in all his naked, tentacled glory it it goes way down. And when the score reaches zero, your character goes stark raving mad.

There are some ways to recover lost sanity points (although spending time in a mental institution tends to do more harm than good), but you tend to lose them faster than you recover them. Call of Cthulhu is probably the only game where you're more likely to end your career in a padded cell than actually die. This tends to encourage extreme caution among players and discourage hoarding of ancient, mystical, manuscripts and artifacts.