Mailing List: An email-based discussion group. Sending one email message to the mailing list sends email to all other members of the group. Mailing lists are usually joined by subscribing, and can be left by unsubscribing.

Masking: To conceal a web site's URL in some manner, normally by using a domain name. For example, if a URL shows up as "http://www.example.com/" but the web site is actually located at "http://www.somewhere-else.com/example/", that URL is said to be "masked".

Megabyte: A million bytes or a thousand (technically 2^10 or 1024) kilobytes.

MIDI: (Musical Instrument Digital Interface) - A high-quality audio file format (.mid).

MIME: (Multipurpose Internet Mail Extensions) - The Internet standard for attaching non-text files to standard email messages. Non-text files can include graphics, spreadsheets, word-processor documents, sound files, etc. An email program is said to be "MIME Compliant" if it can both send and receive files using the MIME standard.

Mirror: To "mirror" something is to maintain an exact copy of it. The most common use of the term on the Internet refers to "mirror sites" which are FTP or web sites that maintain exact copies of material originally stored at another location. Another common use of the term "mirror" refers to writing information to more than one hard disk simultaneously to prevent its loss or destruction.

Modem: (MOdulator, DEModulator) - An electronic device that lets computers communicate with one another, much as telephones work with people. The name is derived from "modulator-demodulator" because of their function in processing data over analog phone lines. Terminal Adapters are often (and mistakenly) referred to as modems.

Mosaic: The first web browser that was available for Macintosh, Windows and UNIX machines with the same interface for each. The popularity of the WWW began with Mosaic.

MPEG: (Motion Picture Experts Group) - A video file format (.mpeg) offering excellent quality in a comparatively small size. Video files found on the Internet are frequently stored in the MPEG format.

Multimedia: A combination of media types in a single document, such as text, graphics, audio and video.

MX Records: MX Records are required to be able to send email to domain names (email@example.com), rather than the actual mail server (email@mailserver.example.com). There are other methods for forwarding messages from a domain to a mail server, but MX Records are the preferred method.