Malware or malicious software is software designed to infiltrate or damage a computer system without the owner's informed consent. It is a portmanteau of the words "malicious" and "software". The expression is a general term used by computer professionals to mean a variety of forms of hostile, intrusive, or annoying software or program code.

Many normal computer users are however still unfamiliar with the term, and most never use it. Instead, "(computer) virus" is used in common parlance and often in the general media to describe all kinds of malware. Another term that has been recently coined for malware is badware, perhaps due to the anti-malware initiative Stopbadware or corruption of the term "malware".

Software is considered malware based on the perceived intent of the creator rather than any particular features. It includes computer viruses, worms, trojan horses, spyware, adware, and other malicious and unwanted software. In law, malware is sometimes known as a computer contaminant, for instance in the legal codes of California, West Virginia, and several other U.S. states.

Malware should not be confused with defective software, that is, software which has a legitimate purpose but contains harmful bugs.