(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.

Alternative form of Mason-Dixon Line

A moment during legal or judicial proceedings when evidence or other information, usually unknown to most present, is introduced into the record in a manner seen as determinative of the outcome of the proceedings regardless of whatever else occurs.

Bonnet-Dechaume-Blanc syndrome

plural of Perry Mason moment

plural of sand mason worm

Lanice conchilega, a burrowing marine polychaete worm.

Alternative form of Mason-Dixon Line

(rare) A female Freemason.