semicircle

noun

1. Also called sem·i·cir·cum·fer·ence [sem-ee-ser-kuhm-fer-uh ns, -fruh ns, sem-ahy-] /ˌsɛm i sərˈkʌm fər əns, -frəns, ˌsɛm aɪ-/. half of a circle; the arc from one end of a diameter to the other.
2. anything having or arranged in the form of a half of a circle.

noun

1. 1. one half of a circle
   2. half the circumference of a circle
2. anything having the shape or form of half a circle

n.1520s, from semi- + circle (n.) or else from Latin semicirculus.