(Usually "e-mail address") The string used to specify the source or destination of an electronic mail message. E.g. "email@example.com".
The RFC 822 standard is probably the most widely used on the Internet. X.400 was once used in Europe and Canada. UUCP-style (bang path) addresses or other kinds of source route became virtually extinct in the 1990s.
In the example above, "john" is the local part which is the name of a mailbox on the destination computer. If the sender and recipient use the same computer, or the same LAN, for electronic mail then the local part is usually all that is required.
If they use different computers, e.g. they work at different companies or use different Internet service providers, then the "host part", e.g. "sales.acme.com" must be appended after an "@". This usually takes the form of a fully qualified domain name or, within a large organisation, it may be just the hostname part, e.g. "sales". The destination computer named by the host part is usually a server of some kind rather than an individual's workstation or PC. The user's mail is stored on the server and read later via client mail software running on the user's computer.
Large organisations, such as universities will often set up a global alias directory which maps a simple user name such as "jsmith" to an address which contains more information such as "firstname.lastname@example.org". This hides the detailed knowledge of where the message will be delivered from the sender, making it much easier to redirect mail if a user leaves or moves to a different department for example.