1. The unauthorized access or compromise of an electronic system, computer, or network by a cyber actor.
Source: Cyber Threats to Elections – A Lexicon, Cyber Threat Intelligence Integration Center & Office of the Director of National Intelligence, https://www.dni.gov/files/CTIIC/documents/CTIIC_2018_Lexicon_without_banner_small_file_for_Post.pdf
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. [In intelligence usage], the recruitment of agents within or the infiltration of agents or technical monitoring devices in an organization or group for the purpose of acquiring information or of influencing its activities. (ICS Glossary). Note: This term was previously in JP 1-02, however rescinded by JP 2-01.2, 11 Mar 2011.
Also, a principal counterintelligence objective is penetration of an adversary, and this can be achieved by the recruitment of a key source within an opponent’s organization. Ideally, the penetration will be the recruitment of a senior figure with sufficient access to compromise all the service’s operations, but lower-level penetrations, such as the management of a double agent, may be sufficient to reveal the identities of case officers and their operational premises. (Historical Dictionary of Cold War Counterintelligence, 2007).
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