It's the opposite of zenith, another word ultimately derived from Arabic. The difference between an apex and a zenith is that an apex exists as a point in spac

It's the opposite of zenith, another word ultimately derived from Arabic. The difference between an apex and a zenith is that an apex exists as a point in space, while a zenith is a direction, with no fixed point which may be said to be "the" zenith. There are other differences given that apex has a few related meanings, but this is the main one.