# speed

An object's average speed is the distance it moves during a given
period of time, divided by the length of the time interval. If you make
the time interval infinitely small, the result is the object's speed at
one particular moment in time. The notion of speed can be applied to waves in different ways; for instance, for a simple wave, the *phase speed* is the speed at which any given wave crest or wave through propagates through space.

Cf. the more general entry velocity.