Nadir always seemed very obviously Arabic to me. Weirdly, I first encountered it in a book on category theory, and only after that did I start to hear it used in everyday English to mean the opposite of 'apex'.
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.