Rigormortis is a postmortem condition characterized by the stiffening of muscles after death. It occurs due to biochemical changes in muscle tissue, particularly the depletion of ATP (adenosine triphosphate), which is necessary for muscle relaxation. Typically, rigor mortis begins within 2 to 6 hours after death, peaks around 12 hours, and gradually dissipates over the following days as the body decomposes. This phenomenon is often used in forensic science to help estimate the time of death.