From the Q&A: What is the Sanity ('SAN') mechanic in Call of Cthulhu?

From our Fan Use and Licensing Q&A:

Q: What is the Sanity ('SAN') mechanic in Call of Cthulhu?

A: Sanity (abbreviated as 'SAN') is the game mechanic in Call of Cthulhu that models the behavior of protagonists in Mythos fiction when confronted with incomprehensible physics and monstrous entities from beyond space and time. The cosmic horrors of the Cthulhu Mythos defy safe or easy comprehension. When faced with such terrors, the human mind attempts to rationalize them but may be corrupted by the experience. The acquisition of Cthulhu Mythos skill points in the game reflects an investigator developing a human-centric understanding of the Mythos, while the loss of SAN points balances this acquisition with the cost associated with this new, but terrible knowledge. In this sense, SAN is a corruption of human morals, behavior, and personality.

"Sanity" is the game’s register of the investigator’s mental resilience. It is not designed to model or make light of real-world mental health conditions, in the same way that hit points (and the loss of them) in most RPGs do not make light of real-world physical trauma and injury.