1530s, "secret, not divulged," from French occulte and directly from Latin occultus "hidden, concealed, secret," past participle of occulere "cover over, conceal," from assimilated form of ob "over" (see ob-) + a verb related to celare "to hide" (from PIE root *kel- (1) "to cover, conceal, save"). Meaning "not apprehended by the mind, beyond the range of understanding" is from 1540s. The association with the supernatural sciences (magic, alchemy, astrology, etc.) dates from 1630s. A verb occult "to keep secret, conceal" (c.1500, from Latin occultare) is obsolete.

early 15c., occultacioun, "disguise or concealment of identity," from Latin occultationem (nominative occultatio), noun of action from past-participle stem of occultare "to hide, conceal," frequentative of occulere "to cover over, conceal" (see occult).

"the doctrine, principles, or practices of occult sciences," 1870, from occult + -ism. Related: Occultist.

Occultism, I should add, is to be distinguished from the primitive magic described by anthropologists, which is prescientific, prephilosophical, and perhaps prereligious, whereas occultism is a pseudo-science or system of pseudo-sciences, often supported by an irrationalist philosophy, and always exploiting the disintegrated débris of preexisting religions. [E.R. Dodds, "The Greeks and the Irrational," 1951, footnote]

Proto-Indo-European root meaning "to cover, conceal, save."

It forms all or part of: Anselm; apocalypse; Brussels; caliology; Calypso; calyx; ceiling; cell; cellar; cellular; cellulite; cellulitis; cilia; clandestine; cojones; coleoptera; color; conceal; eucalyptus; hall; hell; helm (n.2) "a helmet;" helmet; hold (n.2) "space in a ship below the lower deck;" hole; hollow; holster; housing (n.2) "ornamental covering;" hull (n.1) "seed covering;" kil-; kleptomania; occult; rathskeller; supercilious; Valhalla; William.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit cala "hut, house, hall;" Greek kalia "hut, nest," kalyptein "to cover," koleon, koleos "sheath," kelyphos "shell, husk;" Latin cella "small room, store room, hut," celare "to hide, conceal," clam "secret," clepere "to steal, listen secretly to;" Old Irish cuile "cellar," celim "hide," Middle Irish cul "defense, shelter;" Gothic hulistr "covering," Old English heolstor "lurking-hole, cave, covering," Gothic huljan "to cover over," hulundi "hole," hilms "helmet," halja "hell," Old English hol "cave," holu "husk, pod;" Old Prussian au-klipts "hidden;" Old Church Slavonic poklopu "cover, wrapping."

1640s, "secret or occult language," from crypto- "secret, hidden" + -ology. From 1945 as "science of secret characters or codes."

"astrology, art of judging occult influences of stars and planets on human affairs," 1650s; see astro- + -mancy "divination by means of."

1530s, "writing in secret code or occult characters," verbal noun from cipher (v.). Meaning "action of using figures in arithmetic" is from 1610s.

1630s, "hidden, occult, mystical," from Late Latin crypticus, from Greek kryptikos "fit for concealing," from kryptos "hidden" (see crypt). Meaning "mysterious, enigmatic" is attested by 1920. Related: Cryptically.

late 15c., "pertaining to the four elements," from Medieval Latin elementalis, from Latin elementum (see element). Meaning "pertaining to the powers of nature" is from 1823. The noun in the occult sense "a spirit of the elements" is from 1877. Related: Elementally.

also cypher, 1520s, "to do arithmetic" (with Arabic numerals), from cipher (n.). Transitive sense "reckon in figures, cast up" is from 1860. Meaning "to write in code or occult characters" is from 1560s. Related: Ciphered; ciphering.