Speed and Velocity
Speed is the rate of change of distance (scalar); velocity is the rate of change of displacement (vector). Average and instantaneous values are used in kinematics.
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Speed
Speed is the rate at which distance is covered. Average speed = total distance / total time. Speed is a scalar—it has no direction. The SI unit is m/s (metres per second). Instantaneous speed is the speed at a particular instant.
Velocity
Velocity is the rate of change of displacement. It is a vector: it has magnitude (speed) and direction. Average velocity = total displacement / total time. For motion in one dimension (e.g. along the x-axis), we use positive and negative signs to indicate direction. Instantaneous velocity is the velocity at a particular instant.
Uniform velocity
When an object moves with constant velocity, its speed and direction do not change. The displacement in time t is s = v t, where v is the (constant) velocity. Uniform motion is a special case that leads to simple equations and straight-line graphs.
Can an object have constant speed but changing velocity? When?