Source : Webster's Revised Unabridged Dictionary (1913)

Virus \Vi"rus\, n. [L., a slimy liquid, a poisonous liquid,
   poison, stench; akin to Gr. ? poison, Skr. visha. Cf.
   {Wizen}, v. i.]
   1. (Med.)
      (a) Contagious or poisonous matter, as of specific ulcers,
          the bite of snakes, etc.; -- applied to organic
          poisons.
      (b) The special contagion, inappreciable to the senses and
          acting in exceedingly minute quantities, by which a
          disease is introduced into the organism and maintained
          there.

   Note: The specific virus of diseases is now regarded as a
         microscopic living vegetable organism which multiplies
         within the body, and, either by its own action or by
         the associated development of a chemical poison, causes
         the phenomena of the special disease.

   2. Fig.: Any morbid corrupting quality in intellectual or
      moral conditions; something that poisons the mind or the
      soul; as, the virus of obscene books.

Source : WordNet®

virus
     n 1: (virology) ultramicroscopic infectious agent that replicates
          itself only within cells of living hosts; many are
          pathogenic; a piece of nucleic acid (DNA or RNA) wrapped
          in a thin coat of protein
     2: a harmful or corrupting agency; "bigotry is a virus that
        must not be allowed to spread"; "the virus of jealousy is
        latent in everyone"
     3: a software program capable of reproducing itself and usually
        capable of causing great harm to files or other programs
        on the same computer; "a true virus cannot spread to
        another computer without human assistance" [syn: {computer
        virus}]

Source : Free On-Line Dictionary of Computing

virus
     
         (By analogy with biological viruses, via SF) A
        program or piece of code written by a {cracker} that "infects"
        one or more other programs by embedding a copy of itself in
        them, so that they become {Trojan horses}.  When these
        programs are executed, the embedded virus is executed too,
        thus propagating the "infection".  This normally happens
        invisibly to the user.
     
        A virus has an "engine" - code that enables it to propagate
        and optionally a "payload" - what it does apart from
        propagating.  It needs a "host" - the particular hardware and
        software environment on which it can run and a "trigger" - the
        event that starts it running.
     
        Unlike a {worm}, a virus cannot infect other computers without
        assistance.  It is propagated by vectors such as humans
        trading programs with their friends (see {SEX}).  The virus
        may do nothing but propagate itself and then allow the program
        to run normally.  Usually, however, after propagating silently
        for a while, it starts doing things like writing "cute"
        messages on the terminal or playing strange tricks with the
        display (some viruses include {display hacks}).  Viruses
        written by particularly antisocial {crackers} may do
        irreversible damage, like deleting files.
     
        By the 1990s, viruses had become a serious problem, especially
        among {IBM PC} and {Macintosh} users (the lack of security on
        these machines enables viruses to spread easily, even
        infecting the operating system).  The production of special
        {antivirus software} has become an industry, and a number of
        exaggerated media reports have caused outbreaks of near
        hysteria among users.  Many {lusers} tend to blame
        *everything* that doesn't work as they had expected on virus
        attacks.  Accordingly, this sense of "virus" has passed into
        popular usage where it is often incorrectly used for a {worm}
        or {Trojan horse}.
     
        See {boot virus}, {phage}.  Compare {back door}.  See also
        {Unix conspiracy}.
     
        [{Jargon File}]
     
        (2003-06-20)