Daemon

A daemon is a process running in the background (i.e. usually not connected to a controlling physical terminal or pseudo-terminal) which performs some utility task for the system community as a whole. There are now typically innumerable daemons on most systems, many of them handling interactions with a connected data network.

Among the first were line printer spoolers, since a queue of items waiting to be printed might exist. When email appeared, incoming and outgoing messages were handled by email daemons. (Outgoing as well, since the destination might be unresponsive at the exact moment the message was sent, and the daemon would have to keep trying.)