also semi-circle, "the half of a circle," 1520s, from semi- + circle (n.) or else from Latin semicirculus. Also compare semicircular. The meaning "a body or arrangement of objects in a half circle" is by 1590s.
word-formation element meaning "pertaining to, of the nature of," from Latin -arem, -aris "of the kind of, belonging to," a secondary form (by dissimilation) of -alis, used after syllables with an -l- (such as insularis for *insulalis, stellaris for *stellalis).
<a href="https://www.etymonline.com/word/semicircular">Etymology of semicircular by etymonline</a>
Harper, D. (n.d.). Etymology of semicircular. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/semicircular