In the study of physics, non-uniform motion occurs, when an object travels an unequal distance in an equal amount of time. An example would be a car that travels 30 miles during the first hour of a trip and 50 miles during the second hour.
Uniform motion, in contrast, involves an object that covers an equal distance in an equal amount of time. An example might be a train that covers 20 miles per hour for an extended period. Because it may change at any given moment, the velocity of non-uniform motion can only be measured at a specific instant in time.