1. Unauthorized user who attempts to or gains access to an Information System. (CNSS Instruction No. 4009).

2. A person who creates and modifies computer software and hardware, including computer programming, administration, and security-related items. This can be done for either negative or positive reasons. Criminal hackers create malware in order to commit crimes. (McAfee.com, accessed 15 Nov 2010).

3. In computer security usage, a term used for a person who accesses a computer system by circumventing its security system.

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

4. An unauthorized user who attempts to or gains access to an information system.

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

5. Someone who seeks to exploit weaknesses in computer systems, voting systems or networks to gain unauthorized access or break-in into a system. There are many types of hackers, but the best defined terms for types of hackers are white-hat and black-hat hackers.

Source: Information Technology Terminology, U.S. Election Assistance Commission, https://www.eac.gov/documents/2017/09/21/information-technology-terminology-security

6. Unauthorized user who attempts to gain or successfully gains access to an information system.

Source: U.S. Cyberspace Solarium Commission, March 2020, https://subscriber.politicopro.com/f/?id=00000170-c638-d8f7-a7f1-f63b33510000