Define rigor mortis.

Explanation

**Step-by-Step Solution:**

1. **Definition of Rigor Mortis**: Rigor mortis is a postmortem change that occurs after death, characterized by the stiffening of the body's muscles.

2. **Cause of Rigor Mortis**: The stiffening of muscles is due to chemical changes that occur in the myofibrils, which are the contractile units within muscle fibers.

3. **Process of Rigor Mortis**: After death, the body undergoes various biochemical processes. The depletion of ATP (adenosine triphosphate) in muscle cells leads to the inability of muscles to relax, resulting in rigidity.

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