The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

semidiameter
    n 1: the apparent radius of a celestial body when viewed as a
         disc from the earth