Source : Webster's Revised Unabridged Dictionary (1913)

Half-mast \Half"-mast`\, n.
   A point some distance below the top of a mast or staff; as, a
   flag a half-mast (a token of mourning, etc.).

Source : WordNet®

half-mast
     n : a position some distance below the top of the mast to which
         a flag is lowered in mourning or to signal distress