(verb) To send (a copy of) an electronic mail message that you have received on to one or more other addressees. Most e-mail systems can be configured to do this automatically to all or certain messages, e.g. Unix sendmail looks for a ".forward" file in the recipient's home directory.
A mailing list server (or "mail exploder") is designed to forward messages automatically to lists of people.
Unix manual page: aliases(5).