'half-diameter' originates from the combination of 'half' and 'diameter', where 'half' means 'one of two equal parts' and 'diameter' refers to a line passing through the center of a circle or sphere.

The term 'half-diameter' evolved from the need to describe a segment that is half the length of a diameter, eventually becoming a standard term in geometry.

Initially, it meant 'half the length of a diameter', and this meaning has remained consistent in modern usage.