1. A wide range of unwanted programs that exploit infected computers for commercial gain. They can deliver unsolicited pop-up advertisements, steal personal information (including financial information such as credit card numbers), monitor web-browsing activity for marketing purposes, or route HTTP requests to advertising sites. (McAfee.com, accessed 15 Nov 2010).

Source: Terms & Definitions of Interest for DoD Counterintelligence Professionals, Office of the National Counterintelligence, https://www.dni.gov/files/NCSC/documents/ci/CI_Glossary.pdf

2. Software that is secretly or surreptitiously installed into an information system without the knowledge of the system user or owner.

Source: Explore Terms: A Glossary of Common Cybersecurity Terminology, National Initiative for Cybersecurity Careers and Studies (NICCS), https://niccs.us-cert.gov/about-niccs/glossary

3. A kind of malware, which a user unknowingly installs, that can gain access to the computer and steal data.

Source: An Investigation into Foreign Entities Who Are Targeting Servicemembers and Veterans Online, Vietnam Veterans of America, https://vva.org/wp-content/uploads/2019/09/VVA-Investigation.pdf