The measurement of your sanity in the RPG "Call of
Cthulhu". Whenever you see some sanity blasting abomination,
or read out of a blasphemous book like the Necronomicon, you
must make a Sanity Check to see how many SAN points you've
lost. If you loose too many at once, or if your sanity goes too
low, then you'll acquire some kind of mental illness, like a
phobia. If you're SAN score goes all the way down to 0, then
your character goes completely insane, you loose control over him,
and he becomes an NPC.
The term is also used outside of the "Call of Cthulhu" to refer to
someone's sanity, or lack thereof. "Jeez, I knew that Mulder had
lost some SAN points, but still..."