Average velocity is the displacement of an object, divided by the time it took to cover that distance. Displacement is the straight line distance between the starting point and ending point of an object's motion. Velocity is referred to as a vector quantity because it has both magnitude and direction.
When an object travels in a circle it is said to have no velocity because its starting point is the same as its ending point. The ability to measure velocity needs a separate beginning and ending point. The measurements are normally expressed as meters per second, but any distance measurement and time measurement may be substituted.