virus

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of various submicroscopic agents that infect living organisms, often causing disease, and that consist of a single or double strand of RNA or DNA surrounded by a protein coat. Unable to replicate without a host cell, viruses are typically not considered living organisms.
• noun A disease caused by a virus.
• noun A computer program or series of commands that can replicate itself and that spreads by inserting copies of itself into other files or programs which users later transfer to other computers. Viruses usually have a harmful effect, as in erasing all the data on a disk.
• noun A harmful or destructive influence.

from The Century Dictionary.

• noun The contagium of an infectious disease; a poison produced in the body of one suffering from a contagious disease, and capable of exciting the same disease when introduced into another person by inoculation.
• noun Hence Figuratively, that which causes a degraded mental or moral state; moral or intellectual poison: as, the virus of sensuality.
• noun Figuratively, virulence; extreme acrimony or bitterness; malignity.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Med.), Archaic Contagious or poisonous matter, as of specific ulcers, the bite of snakes, etc.; -- applied to organic poisons.
• noun obsolescent the causative agent of a disease, .
• noun 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.
• noun Fig.: Any morbid corrupting quality in intellectual or moral conditions; something that poisons the mind or the soul.
• noun (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}.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun archaic Venom, as produced by a poisonous animal etc.
• noun A type of microscopic agent that causes an infectious disease; the disease so caused.
• noun pathology, microbiology, virology A submicroscopic infectious organism, now understood to be a non-cellular structure consisting of a core of DNA or RNA surrounded by a protein coat. It requires a living cell to replicate, and often causes disease in the host organism.
• noun computing A computer virus.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a software program capable of reproducing itself and usually capable of causing great harm to files or other programs on the same computer
• noun (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
• noun a harmful or corrupting agency