Longman Dictionary of Contemporary English
The Collaborative International Dictionary
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.).
Douglas Harper's Etymology Dictionary
Wiktionary
n. The lowered position, half the height of a mast, at which a flag is flown when mourning, especially expressing respect for the dead.
WordNet
n. a position some distance below the top of the mast to which a flag is lowered in mourning or to signal distress
Wikipedia
Half-mast or half-staff refers to a flag flying below the summit on a pole. In many countries this is seen as a symbol of respect, mourning, distress, or in some cases, a salute.
The tradition of flying the flag at half-mast began in the 17th century. According to some sources, the flag is lowered to make room for an "invisible flag of death" flying above.
When hoisting a flag that is to be displayed at half-mast, it should be hoisted to the finial for an instant, then lowered to half-mast. Likewise, when the flag is lowered at the end of the day, it is to be hoisted to the finial for an instant, and then lowered.
Usage examples of "half-mast".
Of course Fermi could not see him, but he was embarrassed even to be talking to the Italian physicist, a dignified man if ever there was one, with trousers at half-mast.
They must have seen aboard Jade Gate, because the flag went to half-mast.