1. A chunk of computer programming code that makes copies of itself without any concious human intervention. Some viruses do more than simply replicate themselves, they might display messages, install other software or files, delete software of files, etc. A virus requires the presence of some other program to replicate itself. Typically viruses spread by attaching themselves to programs and in some cases files, for example the file formats for Microsoft word processor and spreadsheet programs allow the inclusion of programs called "macros" which can in some cases be a breeding ground for viruses. From Matisse

2. A virus is a program (or a fragment of code) that replicates by attaching a copy of itself to other programs. For a virus to be activated, the software it infects must first be run. Analogy: A biological virus is not a "living" thing. Instead, it is simply a strand of DNA. When it enters a living cell, it takes control of the cell forcing it to generate duplicate copies of the original DNA strand. In much the same way, a computer virus hijacks the computer forcing it to generate duplicate copies of the original virus. Computer viruses are so common because humans do not practice sufficient cyber-hygiene when exchanging files. Key point: An "anti-virus" programs scans the disks on your system hunting down those files that have signatures indicative of infected files. Since file-scanning technology is generic, most anti-virus programs also scan for other hostile content, such as trojans. Contrast: The popular use of the word "virus" means any form of malware. For example, in the movie Office Space, the protagonists write what is called a "virus" that runs in the banking mainframe to steal round-off errors. In contrast, the technical definition limits itself to just those forms of contagious malware that spreads by infecting other programs. Key point: Viruses have a life cycle from the point they are originally created, distributed, found by anti-virus programs, then eradicated. They also mutate as script kiddies take viruses, make small alteration that avoids current virus scanners, and redistribute the viruses. Example: boot sector Historically, the most popular kind of virus, though becoming less popular as floppies are used less often. E.g. Form Virus macro virus Data files cannot contain viruses -- except when they also include scripting "macros". Currently the most popular kind of virus. Most macro viruses are written in VisualBasic, a programming language included as part of Microsoft Office products (Word, Excel). E.g. Marker Virus file infector The traditional definition of a virus: an executable file contains a virus imbedded within. When run, it attaches the virus to other executables on the system. multi-part Uses more than one of the techniques above. toolkits Increasingly, viruses are created via toolkits designed for the purpose of creating viruses that evade anti-virus programs. polymorphic This type of virus automatically changes its form in order to evade anti-virus programs. A common technique is to XOR its code with changing patterns. Culture: Viruses are rarely written by a single human being. Instead, they are often written by groups. Each virus is usually built upon work done by other virus writers. This means that a virus is usually related to other viruses. Computer viruses mutate and exchange genetic material much like biological systems. What we classify as the "author" of a virus is usually somebody who made one small mutation that made a virus especially virulent. From Hacking-Lexicon

3. n. [from the obvious analogy with biological viruses, via SF] A cracker program that searches out other programs and `infects' them 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. 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 nice display hacks). Many nasty viruses, written by particularly perversely minded crackers, do irreversible damage, like nuking all the user's files. In the 1990s, viruses became a serious problem, especially among Windows users; the lack of security on these machines enables viruses to spread easily, even infecting the operating system (Unix machines, by contrast, are immune to such attacks). The production of special anti-virus 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 not only into techspeak but into also popular usage (where it is often incorrectly used to denote a worm or even a Trojan horse). See phage; compare back door; see also Unix conspiracy. From Jargon Dictionary