The Collaborative International Dictionary of English v.0.48:

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.) Contagious or poisonous matter, as of specific
      ulcers, the bite of snakes, etc.; -- applied to organic
      poisons. [Archaic]
      [1913 Webster +PJC]

   2. the causative agent of a disease, . [obsolescent]
      [PJC]

   3. any of numerous submicroscopic complex organic objects
      which have genetic material and may be considered as
      living organisms but have no proper cell membrane, and
      thus cannot by themselves perform metabolic processes,
      requiring entry into a host cell in order to multiply. The
      simplest viruses have no lipid envelope and may be
      considered as complex aggregates of molecules, sometimes
      only a nucleic acid (DNA or RNA) and a coat protein. They
      are sometimes viewed as being on the borderline between
      living and nonliving objects. They are smaller than living
      cells in size, usually between 20 and 300 nm; thus they
      pass through standard filters, and were previously
      referred to as filterable virus. The manifestations of
      disease caused by multiplication of viruses in cells may
      be due to destruction of the cells caused by subversion of
      the cellular metabolic processes by the virus, or by
      synthesis of a virus-specific toxin. Viruses may infect
      animals, plants, or microorganisms; those infecting
      bacteria are also called bacteriophages. Certain
      bacteriophages may be non-destructive and benign in the
      host; -- see bacteriophage.
      [1913 Webster +PJC]

   4. Fig.: Any morbid corrupting quality in intellectual or
      moral conditions; something that poisons the mind or the
      soul; as, the virus of obscene books.
      [1913 Webster]

   5. (Computers) a program or segment of program code that may
      make copies of itself (replicate), attach itself to other
      programs, and perform unwanted actions within a computer;
      also called computer virus or virus program. Such
      programs are almost always introduced into a computer
      without the knowledge or assent of its owner, and are
      often malicious, causing destructive actions such as
      erasing data on disk, but sometime only annoying, causing
      peculiar objects to appear on the display. The form of
      sociopathic mental disease that causes a programmer to
      write such a program has not yet been given a name.
      Compare trojan horse[3].
      [PJC]

WordNet (r) 3.0 (2006):

computer virus
    n 1: 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:
         virus, computer virus]

The Free On-line Dictionary of Computing (30 December 2018):

virus
computer virus
viruses

    (By analogy with biological viruses, via science
   fiction) A program or piece of code, a type of malware,
   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 other
   types of malware such as worms or Trojan horses.

   See boot virus, phage.  Compare back door.  See also
   Unix conspiracy.

   [Jargon File]

   (2003-06-20)