Occult comes from the Latin word occultus (clandestine, hidden, secret), referring to "knowledge of the hidden". In the medical sense it is used to refer to a structure or process that is hidden, e.g. an "occult bleed" may be one detected indirectly by the presence of otherwise unexplained anaemia.
The word has many uses in the English language, popularly meaning "knowledge of the paranormal", as opposed to "knowledge of the measurable", usually referred to as science. The term is sometimes popularly taken to mean "knowledge meant only for certain people" or "knowledge that must be kept hidden", but for most practicing occultists it is simply the study of a deeper spiritual reality that extends beyond pure reason and the physical sciences. The terms esoteric and arcane can have a very similar meaning, and the three terms are often interchangeable.
The term occult is often associated with organizations and orders whose teachings and practices involve secret or magick rights. The name also extends to a large body of alchemical literature and non-Christian spiritual philosophy.