Megahertz (1 000 000 hertz). A measurement unit of radio frequency, oscillation and vibration, equaling 1 000 000 cycles per second.

Maritime Identification Digit. A 3 digit decimal number used as the first part of ship station identity (MMSI) to indicate nationality.

Minimum Keyboard and Display. A minimal text only display provided for AIS Class–A transceivers.

Maritime Mobile Service Identity. The number used to identify coast stations’ and ships’ DSC, NBDP and AIS systems.

Mobile Packet Data Service. An Inmarsat service allowing continuous connection of ships to terrestrial networks with payment for volume of data exchanged, rather than the duration of ‘airtime’ used.

Maritime Rescue Coordination Centre. The Australian MRCC is located in Canberra and operated by AMSA (see also RCC and JRCC).

Marine Radio Operator’s Certificate of Proficiency. Replaced by Long Range Operator Certificate of proficiency.

Marine Radio Operator’s VHF Certificate of Proficiency. Replaced by Short Range Operator Certificate of Proficiency.

Maritime Safety Committee of the IMO.

Maritime Safety Information. Navigational and meteorological warnings, meteorological forecasts and other urgent safety related messages.