The Collaborative International Dictionary of English v.0.48:

Influenza \In`flu*en"za\, n. [It. influenza influence, an
   epidemic formerly attributed by astrologers to the influence
   of the heavenly bodies, influenza. See Influence.] (Med.)
   An epidemic viral infectious disease characterized by acute
   nasal catarrh, or by inflammation of the throat or the
   bronchi, and usually accompanied by fever and general
   weakness; also called grippe. It is caused by several forms
   of RNA virus which mutate readily and thereby render vaccines
   prepared against older forms ineffective, often requiring a
   new form of vaccine for each new outbreak.
   [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

influenza
    n 1: an acute febrile highly contagious viral disease [syn:
         influenza, flu, grippe]