The word occult originated from the Latin word occultus (clandestine, hidden, secret). It refers to secret or hidden knowledge. The term is most commonly associated with practices involving witchcraft and magick, demonology, etc.
In astronomy an occult means an eclipse (partial or total) of a celestial body. An occultation occurs when one celestial object is hidden by another celestial object passing in front of it.
In medicine the term occult can refer to something that is hidden, e.g. an occult bleed.