half-mast
noun uk/ˌhɑːfˈmɑːst/ us/ˌhæfˈmæst/ (US also half-staff)
at half-mast 下半旗（以示对某人的哀悼）
A flag that is at half mast has been brought down to a point halfway down the pole as an expression of sadness at someone's death.
The flags at Buckingham Palace were all flying at half-mast. 白金汉宫的旗帜全都降了半旗。