The Collaborative International Dictionary of English v.0.48:

Shaster \Shas"ter\, Shastra \Shas"tra\, n. [Skr. c[=a]stra an
   order or command, a sacred book, fr. c[=a]s to order,
   instruct, govern. Cf. Sastra.]
   A treatise for authoritative instruction among the Hindoos; a
   book of institutes; especially, a treatise explaining the
   Vedas. [Written also sastra.]
   [1913 Webster]