Source : Webster's Revised Unabridged Dictionary (1913)

Semidiameter \Sem`i*di*am"e*ter\, n. (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.

Source : WordNet®

semidiameter
     n : the apparent radius of a celestial body when viewed as a
         disc from the earth