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

Moby Thesaurus II by Grady Ward, 1.0:

62 Moby Thesaurus words for "Vishnu":
   Agni, Asvins, Atman, Avalokita, Bhairava, Bhairavi, Brahma, Buddha,
   Chandi, Daksha, Devaki, Devi, Dharma, Dharti Mai, Durga, Dyaus,
   Ganapati, Ganesa, Ganesha, Garuda, Hanuman, Himavat, Indra,
   Jagannath, Juggernaut, Kali, Kama, Kamsa, Karttikeya, Kurma,
   Lakshmi, Matsya, Narsinh, Parjanya, Parshuram, Parvati, Pushan,
   Rahu, Rama, Rhibhus, Rudra, Sarasvati, Savitar, Shiva, Sita, Siva,
   Soma, Surya, Uma, Ushas, Vaja, Vaman, Varah, Varuna, Varuni, Vayu,
   Yama, the Destroyer, the Preserver, the Regenerator,
   the Supreme Soul, the Universal Ego