Computers
Latency "is a measure of the amount of time between a request and a response."[1]
Telecommunications
Latency is “the amount of time it takes a packet to travel from source to destination.”[2] Latency is affected by physical distance, the number of “hops” from one internet network to another internet network that must be made to deliver the packets (since there can be congestion at each hand-off point), and voice-to-data conversion.
Network technologies that create significant time delay can arguably degrade the performance of many interactive Internet applications. The extent of the degradation increases with the extent of the delay. Latency is particularly important for voice applications such as VoIP and Video Relay Service (VRS) where a high degree of latency can degrade voice communication to an unintelligible level. Other non-voice, interactive Internet applications may also be less tolerant of the effects of latency. These include some educational applications, some telework applications, telepresence, many telemedicine applications, and interactive online gaming.
Together, latency and bandwidth define the speed and capacity of a network.