something hidden, secret, or invisible. The occult sciences are magic, necromancy, cabbala, &c. Occult qualities, in philosophy, were those qualities of body or spirit which baffled the investigation of philosophers, and for which they were unable to give any reason; unwilling, however, to acknowledge their ignorance, they deceived themselves and the vulgar by an empty title, calling what they did not know occult.

in Geometry, is used for a line that is scarce perceivable, drawn with the point of the compasses or a leaden pencil. These lines are used in several operations, as the raising of plans, designs of buildings, pieces of perspective, &c. They are to be effaced when the work is finished.