The Collaborative International Dictionary of English v.0.48:

Abraxas \A*brax"as\, n. [A name adopted by the Egyptian Gnostic
   Basilides, containing the Greek letters [alpha], [beta],
   [rho], [alpha], [xi], [alpha], [sigma], which, as numerals,
   amounted to 365. It was used to signify the supreme deity as
   ruler of the 365 heavens of his system.]
   A mystical word used as a charm and engraved on gems among
   the ancients; also, a gem stone thus engraved.
   [1913 Webster]