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an ultramicroscopic (20 to 300 nm in diameter), metabolically inert, infectious agent that replicates only within the cells of living hosts, mainly bacteria, plants, and animals: composed of an RNA or DNA core, a protein coat, and, in more complex types, a surrounding envelope.
Informal. a disease.
a corrupting influence on morals or the intellect; poison.
a segment of self-replicating code planted illegally in a computer program, often to damage or shut down a system or network.
Contemporary Examples

Basically, the malware presents itself as a piece of antivirus software that is going to protect your machine.
Is Your Mac Safe? Dan Lyons June 20, 2011

And most users don’t need to install things like antivirus software that’s almost a necessity on Windows.
Is Your Mac Safe? Dan Lyons June 20, 2011

noun
(modifier) a piece of software designed to prevent viruses entering a computer system or network: antivirus software
(modifier) of or relating to such a piece of software
noun (pl) -ruses
any of a group of submicroscopic entities consisting of a single nucleic acid chain surrounded by a protein coat and capable of replication only within the cells of living organisms: many are pathogenic
(informal) a disease caused by a virus
any corrupting or infecting influence
(computing) an unauthorized program that inserts itself into a computer system and then propagates itself to other computers via networks or disks; when activated it interferes with the operation of the computer
n.

1903, from anti- + virus.
n.

late 14c., “venomous substance,” from Latin virus “poison, sap of plants, slimy liquid,” probably from PIE root *weis- “to melt away, to flow,” used of foul or malodorous fluids, with specialization in some languages to “poisonous fluid” (cf. Sanskrit visam “poison,” visah “poisonous;” Avestan vish- “poison;” Latin viscum “sticky substance, birdlime;” Greek ios “poison,” ixos “mistletoe, birdlime; Old Church Slavonic višnja “cherry;” Old Irish fi “poison;” Welsh gwy “fluid, water,” gwyar “blood”). Main modern meaning “agent that causes infectious disease” first recorded 1728. The computer sense is from 1972.

virus vi·rus (vī’rəs)
n. pl. vi·rus·es

Any of various simple submicroscopic parasites of plants, animals, and bacteria that often cause disease and that consist essentially of a core of RNA or DNA surrounded by a protein coat. Unable to replicate without a host cell, viruses are typically not considered living organisms.

A disease caused by a virus.

virus
(vī’rəs)
Plural viruses

Any of various extremely small, often disease-causing agents consisting of a particle (the virion), containing a segment of RNA or DNA within a protein coat known as a capsid. Viruses are not technically considered living organisms because they are devoid of biological processes (such as metabolism and respiration) and cannot reproduce on their own but require a living cell (of a plant, animal, or bacterium) to make more viruses. Viruses reproduce first either by injecting their genetic material into the host cell or by fully entering the cell and shedding their protein coat. The genetic material may then be incorporated into the cell’s own genome or remain in the cytoplasm. Eventually the viral genes instruct the cell to produce new viruses, which often cause the cell to die upon their exit. Rather than being primordial forms of life, viruses probably evolved from rogue pieces of cellular nucleic acids. The common cold, influenza, chickenpox, smallpox, measles, mumps, yellow fever, hemorrhagic fevers, and some cancers are among the diseases caused by viruses.

Computer Science A computer program that duplicates itself in a manner that is harmful to normal computer use. Most viruses work by attaching themselves to another program. The amount of damage varies; viruses may erase all data or do nothing but reproduce themselves.

viral adjective
virus [(veye-ruhs)]

plur. viruses

Microorganisms consisting of DNA and RNA molecules wrapped in a protective coating of proteins. Viruses are the most primitive form of life. They depend on other living cells for their reproduction and growth. (See under “Medicine and Health.”)

Note: Viruses cause many diseases. (See viral infection.)

virus [(veye-ruhs)]

plur. viruses

A minute organism that consists of a core of nucleic acid surrounded by protein. Viruses, which are so small that a special kind of microscope is needed to view them, can grow and reproduce only inside living cells. (See under “Life Sciences.”)

See computer virus.

antivirus software