TTL
Time To Live (TTL) can refer to either the lifetime of a packet in a network, or the expiry time of cached data.
Networking
In networking, the TTL, embedded in the packet, is a usually defined as a number of hops or as an expiration timestamp after which the packet is dropped. It provides a way to avoid network congestion, but releasing packets after they roamed the network too long.
Caching
In the context of caching, TTL (as an unsigned 32-bit integer) being a part of the HTTP response header or the DNS query, indicates the amount of time in seconds during which the resource can be cached by the requester.