Flag at half-mast and Half-staff

Half-staff and flag at half-mast are semantically related. In some cases you can use "Half-staff" instead a noun phrase "Flag at half-mast".

Half-staff noun - A position some distance below the top of a mast to which a flag is lowered in mourning or to signal distress.

Flag at half-mast and half-staff are semantically related. You can use "Flag at half-mast" instead a noun phrase "Half-staff".