For moving head system, there are two components involved in the time delay between receiving an address and the beginning of the actual data transfer.

Seek Time:

Seek time is the time required to move the read/write head to the proper track. This depends on the initial position of the head relative to the track specified in the address.

Rotational Delay:

Rotational delay, also called the latency time is the amount of time that elapses after the head is positioned over the correct track until the starting position of the addressed sector comes under the Read/write head.