1. In mechanics, a body which does not change its shape or size regardless of the force applied to it; that is, the relative position of its component particles is absolutely fixed in positon relative to each other.
2. An actual body whose behavior approaches that of an ideal rigid body; such as, a steel beam.
3. An idealized extended solid whose size and shape are definitely fixed and remain unaltered when forces are applied.

The rigid body assumption is a mathematical convenience that is useful and gives correct results for many important phenomena.