Acceleration is generally defined as the time rate of change of velocity. When can it be defined as the time rate of change of speed?
When moving in a straight line, the time rate of change of speed is called velocity.
When moving in a straight line, the time rate of change of speed is acceleration.
When moving in a circle, the time rate of change of speed is always zero.
When moving in a circle, the time rate of change of speed is called average speed.