[Common] Malicious software. Software intended to cause consequences the
unwitting user would not choose; especially used of virus or Trojan
The Free On-line Dictionary of Computing (30 December 2018):
Any software designed to do something that the
user would not wish it to do, hasn't asked it to do, and often
has no knowledge of until it's too late. Types of malware
include backdoor, virus, worm, Trojan horse.
Malware typically affects the system on which it is run,
e.g. by deleting or corrupting files on the local disks.
Since Internet connections became common, malware has
increasingly targeted remote systems. An early example was
malware consisting of a malicious e-mail attachment that
targeted security flaws in Microsoft Outlook (the most
common e-mail client) to send itself to all the user's
contacts. A more recent kind of malware "recruits" the
infected computer to become part of a botnet consisting of
thousands of infected computers that can then be remotely
controlled and used to launch DDoS attacks.