Half-mast or half-staff (American English) refers to a flag flying below the summit of a ship mast, a pole on land, or a pole on a building. In many countries this is seen as a symbol of respect, mourning, distress, or, in some cases, a salute. Most English-speaking countries use the term half-mast in all instances. In the United States, this refers officially only to flags flown on ships, with half-staff used on land.

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. However, there is disagreement about where on a flagpole a flag should be when it is at half-mast. It is often recommended that a flag at half-mast be lowered only as much as the hoist, or width, of the flag. British flag protocol is that a flag should be flown no less than two-thirds of the way up the flagpole, with at least the height of the flag between the top of the flag and the top of the pole. It is common for the phrase to be taken literally and for a flag to be flown only halfway up a flagpole, although some authorities deprecate that practice.

When hoisting a flag that is to be displayed at half-mast, it should be raised to the finial of the pole for an instant, then lowered to half-mast. Likewise, when the flag is lowered at the end of the day, it should be hoisted to the finial for an instant, and then lowered.