Sanity
Sanity, in roleplaying games, is a measure of how mentally stable a character is. In traditional roleplaying games, such as AD&D, a character's mental state was entirely up to how the character's player wished to portray them; however, Call of Cthulhu, based on the Cthulhu Mythos by writer H.P. Lovecraft, features the progression to insanity as a major feature of the setting and gameplay, and introduced the Sanity attribute to track the character's descent into madness. Players would "roll for SAN loss" every time they experienced something unpleasant or unsettling, like seeing a corpse, or reading the Necronomicon; if they failed to roll under their current Sanity, they lose some random number of SAN points, and lose a smaller number (or none at all) if they succeed, eventually becoming a gibbering and/or villainous NPC when their Sanity reaches zero. The concept proved extremely popular, and has since been modified and incorporated into a wide variety of different RPGs, either as a core part of the system or as an optional rule for gamers running horror-based campaigns.
One of the most notable updates of the Sanity mechanic (which was something like mental hit points in older games) has been carried out by games such as Unknown Armies. UA sanity works in such a way that, while it can make you more used to seeing horrible things (becoming "hardened" in respect to some horror, like eviscerated bodies), it also makes you colder, insensitive and so on (more likely to carry out violent deeds), while in Call of Cthulhu you just kept losing your sanity points until you went crazy and had to be institutionalized.
Dark Heresy has an insanity rating, that functions similarly. Players start at 0 insanity, and go up from there. Insanity can be gained from psyker or demonic powers, failing a fear test, or if the DM just wants to give some to the player. There is usually a willpower test in order to avoid taking the insanity damage. Every 10 points of insanity, a player rolls a willpower test, and, should he fail it, the rolls on a Mental Trauma table that imposes a temporary penalty. Upon reaching 40, 60, and 80 insanity, the GM gives the players a mental disorder, such as a phobia, visions, nightmares, delusions, etc. At 100 insanity, a player is terminally insane and is removed from play (a houserule is to allow a character to burn fate to regain sanity). Gaining insanity is not a bad thing, because if the first digit of your insanity total is double or more a thing's fear rating, you are immune to its petty horrors. Dark Heresy's Corruption mechanic works similarly, but is a reflection of how tainted your character is by the Warp. Adeptus Evangelion inherits this Insanity system from Dark Heresy.
The system has also been stolen by clueless game designers who fail to understand that it's hard to believe your character is looking at something that is making them go insane when you, the player, are looking at the same thing and it's just kinda funky looking.