(chemistry) A unit of mass equal to 1/12 the mass of an atom of the ¹²C isotope of carbon.

(nautical) The top of a mast. / (US) A list of a newspaper or other periodical's main staff, contributing writers, publisher, circulation, advertising rates etc.

(transitive, nautical) To send to the masthead as a punishment. / (transitive) To furnish (a newspaper) with a masthead.

Abbreviation of Massachusetts.

(computing) having many individual or distributed processing units and thus able to carry out many simultaneous calculations

The position or authority of a master; dominion; command; supremacy; superiority. / Superiority in war or competition; victory; triumph; preeminence.

Acronym of Mobile Army Surgical Hospital.

A key designed to open a set of several locks; a passkey. / (figuratively) Any tool or technique that can solve many types of difficulty.

Alternative letter-case form of Mason-Dixon Line

(historical) The boundary line between Pennsylvania and Maryland, as run (1764–1767) by Charles Mason and Jeremiah Dixon, that, before abolition, defined part of the northern boundary of states in which slavery was permitted.