What is occult ?

The word occult is derived from the Latin word occultus which means "hidden". Occult blood is a medical term which refers to blood which cannot be seen, except perhaps under a microscope. The Occult has been defined as any activity which

1. is esoteric (employs knowledge that is not known to the general public, but which is only revealed gradually to a selected few in training), and
2. depends upon those talents which lie beyond the five senses, and
3. engages with the supernatural.

Unfortunately, such a definition has serious flaws:
bullet The beliefs, rituals, and practices of many activities which have traditionally been considered to be occultic have now been thoroughly documented and published. The public can go to a bookstore and purchase written material on just about any "occultic: activity. Little remains secret. 1 For example, most lists of occultic activity includes Wicca (often referred to by the public as "White" Witchcraft), and various methods of foretelling the future (e.g. astrology, casting runes, reading Tarot Cards, 5 I Ching, etc.). Literally hundreds of books have been published on these topics. Anyone can learn as much as they wish by simply buying or borrowing books. Since knowledge is not hidden, these beliefs and activities do not meet the first criteria above.
bullet Consider a person who joins an adult religion class at their local church and is taught special techniques of effective prayer -- methods that are unique to that church and not generally available to outsiders. This activity would meet all of the three criteria for the Occult: it has to be taught; it can be a silent prayer - a means of communicating without spoken words; it is directed at God.
bullet Consider a ceremonial magician performing a healing spell. She/he might believe that the spell taps into perfectly natural forces - parts of the universe that science has not yet been able to explain. They might feel that they are not part of the Occult, since they are not dealing with the supernatural.
bullet During mass, a Roman Catholic priest will perform a ritual which involves certain words and actions. The end result is that the wafer and wine is believed to literally become the body and blood of Jesus Christ. This act of magic would meet these three criteria, and thus be occultic.
bullet The Wiccan faith is a reconstructed religion of the ancient Celts. Wiccans develop an intimate connection with their Goddess and God, perform religious rituals, and observe seasonal days of celebration. In doing so, they differ little from followers of other religions. Although many Wiccans also engage in divination and/or healing spells, some do not. Among those who do not, their faith is devoid of occultic components.

The rather simple definition:

OCCULT: A set of mostly unrelated divination and/or spiritual practices or activities which appear to tap into forces that have not been explained by science, and which are not conventional practices seen in traditional religions.