1. A collection of computers compromised by malicious code and controlled across a network. A programmed Internet-connected device that can be used to launch DDOS attacks [Distributed Denial of Service (DDoS) attacks], steal data, send spam, etc. Bots are frequently spread as email attachments and can compromise election office computers used to browse websites and support email activities.
Source: Glossary - Introduction to Information Technology for Election Officials, U.S. Election Assistance Commission, https://www.eac.gov/assets/1/28/Glossary_IT-Terms_Managing_Election_Technology.pdf
2. A collection of zombie PCs [personal computers]. Botnet is short for robot network. A botnet can consist of tens or even hundreds of thousands of zombie computers. A single PC in a botnet can automatically send thousands of spam messages per day. The most common spam messages come from zombie computers. (McAfee Labs - Threat Glossary) .
Also, Botnets, or Bot Networks, are made up of vast numbers of compromised computers that have been infected with malicious code, and can be remotely-controlled through commands sent via the Internet. (CRS Report PL32114, 29 Jan 2008).
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
3. A collection of computers infected with malicious bots. These can be fully remotely controlled by the attacker (the owner of the botnet). Depending on its size, a botnet may consist of several hundred to millions of compromised computers
Source: Information Assurance Situation in Switzerland and Internationally, Reporting and Analysis Centre for Information Assurance MELANI, https://www.newsd.admin.ch/newsd/message/attachments/11945.pdf
4. A collection of computers compromised by malicious code and controlled across a network without the owner’s knowledge or consent. (NICCS).
Source: The Cyber Glossary, National Security Archive, George Washington University, https://nsarchive.gwu.edu/news/cyber-vault/2018-09-19/cyber-glossary
5. A network of compromised computers (or bots) under unified command and control.
Source: U.S. Cyberspace Solarium Commission, March 2020, https://subscriber.politicopro.com/f/?id=00000170-c638-d8f7-a7f1-f63b33510000