Half mast and Half-staff

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

Half mast and half-staff are semantically related. Sometimes you can replace phrase "Half mast" with "Half-staff", this phrases are similar.