Buddhachhâyâ (Sk.). Lit., “the shadow of Buddha”. It is said to become visible at certain great events, and during some imposing ceremonies performed at Temples in commemoration of glorious acts of Buddhas life. Hiouen-tseng, the Chinese traveller, names a certain cave where it occasionally appears on the wall, but adds that only he whose mind is perfectly pure”, can see it.
Source: H.P.Blavatsky - The Theosophical Glossary