Source : Webster's Revised Unabridged Dictionary (1913)

Vishnu \Vish"nu\, n. [Skr. Vish?u, from vish to pervade., to
   extend through nature.] (Hindu Myth.)
   A divinity of the modern Hindu trimurti, or trinity. He is
   regarded as the preserver, while Brahma is the creator, and
   Siva the destroyer of the creation.

Source : WordNet®

Vishnu
     n : the Sustainer; a Hindu divinity worshipped as the preserver
         of worlds