The period of time that elapses between a request for information from disk or memory and the arrival of that information at the requesting device.
Memory-access time refers to the time it takes to transfer a character between memory and the processor. Disk-access time refers to the time it takes to place the read/write heads over the requested data. RAM may have an access time of 80 nanoseconds or less, while hard-disk access time could be 10 milliseconds or less.