Timeout

In a packet switching communication network, a timeout occurs when no acknowledgement for data or a packet sent by an entity to another is received from the other before a given time after it was originally sent - presumably because it was damaged or lost by the network. After the timeout happens, the data/packet will be retransmitted.

Since the delay is a real-time delay (since the entity at the other end is independent), such systems are a variety of real-time system. In a wide area network, calculating what the appropriate delay should be is a non-trivial problem, and it took well over a decade of work before the timers in the TCP/IP protocol suite worked well.