The Collaborative International Dictionary of English v.0.48:

Vishnu \Vish"nu\ (v[i^]sh"n[=oo]), n. [Skr. Vish[.n]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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Vishnu
    n 1: the sustainer; a Hindu divinity worshipped as the preserver
         of worlds