1. Virus is 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. It is unable to replicate without a host cell, viruses are typically not considered living organisms.
2. 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.
(2008, Geary W.Skick. Protecting your business in a pandemic : plans, tools, and advice for maintaining business continuity. ISBN: 978-0-313-34602-6)
3. An unauthorised programme that inserts itself into a computer system and then propagates itself to other computers via networks or disks.
(Source: Business Continuity Institute - BCI)
(Source: ENISA - the European Network and Information Security Agency. BCM & Resilience Glossary)
3. A virus is an extremely tiny infectious agent that is only able to live inside a cell. Basically, viruses are composed of just two parts. The outer part is a protective shell made of protein. This shell is often surrounded by another protective layer or envelope, made of protein or lipids (fats). The inner part is made of genetic material, either RNA or DNA. A virus does not have any other structures (called organelles) that living cells have, like a nucleus or mitochondria. These organelles are the tiny organs that maintain a cell's metabolism (life processes). A virus has no metabolism at all. Because a virus lacks organelles, it cannot reproduce itself by itself. To reproduce, it invades a cell within the body of a human or other creature, called the host. Each type of virus has particular types of host creatures and host cells that it will invade successfully. Once within the host cell, the virus uses the cell's own organelles to produce more viruses. In essence, the virus forces the cell to replicate the virus' own genetic material and protective shell. Once replicated, the new viruses leave the host cell and are ready to invade others.
(Source: Centers for Disease Control and Prevention)