Software that attempts to protect a device from being taken over by malicious software (or “malware"). Viruses were some of the first and most prevalent forms of malware; they were named viruses to reflect the way they would spread from device to device. These days most antivirus software concentrate on warning you if you look to be downloading a suspicious file from an external source, and examining files on your computer to see if they match the software's idea of what malware looks like.
Anti-virus software can only recognize malware if it is substantially similar to samples that the anti-virus developer has already analyzed. This makes it far less effective at combating targeted malware designed to infiltrate a particular community or person, rather than more widespread strains of malware. Some advanced malware can also actively attack or conceal itself from antivirus software.