The Collaborative International Dictionary of English v.0.48:

Semicircle \Sem"i*cir`cle\, n.
   1.
      (a) The half of a circle; the part of a circle bounded by
          its diameter and half of its circumference.
      (b) A semicircumference.
          [1913 Webster]

   2. A body in the form of half of a circle, or half of a
      circumference.
      [1913 Webster]

   3. An instrument for measuring angles.
      [1913 Webster]

WordNet (r) 3.0 (2006):

semicircle
    n 1: a plane figure with the shape of half a circle [syn:
         semicircle, hemicycle]

Moby Thesaurus II by Grady Ward, 1.0:

22 Moby Thesaurus words for "semicircle":
   crescent, crescent moon, fifty percent, fifty-fifty, half,
   half circle, half-and-half, half-moon, hemicycle, hemisphere,
   horseshoe, lunula, lunule, mediety, meniscus, moiety, quadrant,
   scythe, sector, semisphere, sextant, sickle