semidiameter Meaning, Definition & Usage

  1. noun the apparent radius of a celestial body when viewed as a disc from the earth

WordNet

Sem`i*di*am"e*ter noun
Definitions
  1. (Math.) Half of a diameter; a right line, or the length of a right line, drawn from the center of a circle, a sphere, or other curved figure, to its circumference or periphery; a radius.

Webster 1913