The Collaborative International Dictionary of English v.0.48:

Gooroo \Goo*roo"\, Guru \Gu*ru"\ (g[=oo]*r[=oo]"), n. [Hind.
   gur[=u] a spiritual parent or teacher, Skr. guru heavy,
   noble, venerable, teacher. Cf. Grief.]
   1. A spiritual teacher, guide, or confessor among the Hindus;
      a guru. --Malcom.
      [1913 Webster]

   2. (g[=oo]"r[=oo]) one who has expert knowledge of a
      technical area and serves as an advisor to others; an
      expert and teacher. Usually written guru.
      [PJC]

   3. (g[=oo]"r[=oo]) an intellectual or spiritual guide or
      advisor; a mentor. Usually written guru.
      [PJC]