Hybrid threats refer to the ability of adversaries—lone attackers, criminal, transnational terrorist organizations, even nation-states—to employ combinations of tactics, technologies, and capabilities to gin an asymmetric advantage. (Quadrennial Homeland Security Review Report, Feb 2010). Also, a hybrid threat is the diverse and dynamic combination of regular forces, irregular forces, criminal elements, or a combination of these forces and elements all unified to achieve mutually benefitting effects. (Army FM 3-0, Operations, Feb 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