The Collaborative International Dictionary of English v.0.48:
Half-mast \Half"-mast`\ (-m[.a]st`), n. A point some distance below the top of a mast or staff; as, a flag a half-mast (a token of mourning, etc.). [1913 Webster]WordNet (r) 3.0 (2006):
half-mast n 1: a position some distance below the top of a mast to which a flag is lowered in mourning or to signal distress [syn: half-mast, half-staff]