rigor mortis
the rigid stiffening of skeletal and cardiac muscle shortly after death.

Stiffness of a cadaver. The following are crude approximations of the time of death, based on temperature and presence/absence of rigor
warm and flaccid < 3 hours
warm and stiff 3-8 hours
cold and stiff 8-36 hours [Molycorpse's current state as of 1/23/13)
cold and flaccid > 36 hours