(Often shortened in context to "list") An electronic mail address that is an alias (or macro, though that word is never used in this connection) which is expanded by a mail exploder to yield many other e-mail addresses. Some mailing lists are simple "reflectors", redirecting mail sent to them to the list of recipients. Others are filtered by humans or programs of varying degrees of sophistication; lists filtered by humans are said to be "moderated".
Mailing lists are one of the primary forms of hacker interaction, along with Usenet. They predate Usenet, having originated with the first UUCP and ARPANET connections. They are often used for private information-sharing on topics that would be too specialised for or inappropriate to public Usenet groups. Though some of these maintain almost purely technical content (such as the Internet Engineering Task Force mailing list), others (like the "sf-lovers" list maintained for many years by Saul Jaffe) are recreational, and many are purely social. Perhaps the most infamous of the social lists was the eccentric bandykin distribution; its latter-day progeny, lectroids and tanstaafl, still include a number of the oddest and most interesting people in hackerdom.
Mailing lists are easy to create and (unlike Usenet) don't tie up a significant amount of machine resources (until they get very large, at which point they can become interesting torture tests for mail software). Thus, they are often created temporarily by working groups, the members of which can then collaborate on a project without ever needing to meet face-to-face.
Requests to subscribe to, or leave, a mailing list should ALWAYS be sent to the list's "-request" address (e.g. email@example.com for the IETF mailing list). This prevents them being sent to all recipients of the list and ensures that they reach the maintainer of the list, who may not actually read the list.