Source : Webster's Revised Unabridged Dictionary (1913)

Gooroo \Goo*roo"\, Guru \Gu*ru"\, n. [Hind. gur? a spiritual
   parent or teacher, Skr. guru heavy, noble, venerable,
   teacher. Cf. {Grief}.]
   A spiritual teacher, guide, or confessor amoung the Hindoos.
   --Malcom.

Source : WordNet®

guru
     n 1: a Hindu or Buddhist religious leader and spiritual teacher
     2: each of the first ten leaders of the Sikh religion
     3: a recognized leader in some field or of some movement; "a
        guru of genomics"

Source : Free On-Line Dictionary of Computing

guru
     
         An expert, especially in "{Unix} guru".  Implies not
        only {wizard} skill but also a history of being a knowledge
        resource for others.  Less often, used (with a qualifier) for
        other experts on other systems, as in "VMS guru".
     
        See {source of all good bits}.
     
        [{Jargon File}]
     
        (1996-06-01)