Latency is a term used in various computing areas,

a) Computer systems - refers to the perceived delay it takes a system to access data. It can be defined as the period it takes a hard disk etc. to read and write the proper sectors required. There are numerous contributors to system latency, such as a mismatch in CPU and input/output devices.

b) Networking - refers to the perceived delay it takes a packet of data to travel to it's destination from the source. Common causes include optical fiber cables, wireless transmission obstacles, router nodes and storage delays from switches and bridges.

Due to the daily nature of delays in networking, latency is perhaps a phrase more commonly used in connection with networking than computer systems.