Gnosis (Gr.) Lit. "knowledge." The technical term used by the schools of religious philosophy, both before and during the first centuries of so-called Christianity, to denote the object of their enquiry. This spiritual and sacred knowledge, the Gupta Vidya of the Hindus, could only be obtained by Initiation into Spiritual Mysteries of which the ceremonial "Mysteries" were a type.
Source: H.P.Blavatsky - The Key to Theosophy, The theosophical glossary