The delay between requesting and accessing data. For disk drives it refers to the delay due to disk rotation only, even though this is only one of several factors that determines time to access data from disks. The faster a disk spins the sooner it will be at the position where the required data is under the replay head. As disk diameters have decreased so rotational (spindle) speeds have tended to increase but there is still much variation. Modern 2.5-inch drives typically have spindle speeds of between 7,200 and 15,000 RPM, so one revolution is completed in 8 to 4 ms respectively. This is represented in the disk specification as average latency of 4 or 2 ms.
For solid-state ‘drives’ (SSD) the latency is much less resulting in faster access to data, particularly if the data is scattered around the store; here, SSD can be over 100 times faster than HDD, but just a few times faster for large unscattered data files.