In computing, a daemon is a program that runs as a background process, rather than being under the direct control of an interactive user. Customary convention is to name a daemon process with the letter d as a suffix to indicate that it's a daemon. For example, syslogd is a daemon that implements system logging facility, and sshd is a daemon that serves incoming SSH connections.
Even though the concept can apply to many computing systems, the term daemon is used almost exclusively in the context of Unix-based systems. In other contexts, different terms are used for the same concept.