aorist (n.)

prefix meaning "not, without," from Greek a-, an- "not" (the "alpha privative"), from PIE root *ne- "not" (source also of English un-).

In words from Greek, such as abysmal, adamant, amethyst; also partly nativized as a prefix of negation (asexual, amoral, agnostic). The ancient alpha privatum, denoting want or absence.

Greek also had an alpha copulativum, a- or ha-, expressing union or likeness, which is the a- expressing "together" in acolyte, acoustic, Adelphi, etc. It is from PIE root *sem- (1) "one; as one, together with."

late 14c., orisoun, from Old French orizon (14c., Modern French horizon), earlier orizonte (13c.), from Latin horizontem (nominative horizon), from Greek horizon (kyklos) "bounding (circle)," from horizein "bound, limit, divide, separate," from horos "boundary, landmark, marking stones." The h- was restored in English 17c. in imitation of Latin. Old English used eaggemearc ("eye-mark") for "limit of view, horizon." The apparent horizon is distinguished from the celestial or astronomical horizon.