Hybrid warfare was defined by Frank Hoffman in 2007 as the emerging simultaneous use of multiple types of warfare by flexible and sophisticated adversaries who understand that successful conflict requires a variety of forms designed to fit the goals at the time. A US document on maritime strategy said "Conflicts are increasingly characterized by a hybrid blend of traditional and irregular tactics, decentralized planning and execution, and non-state actors using both simple and sophisticated technologies in innovative ways." While there is no clear, accepted definition, methods include political warfare and blend conventional warfare, irregular warfare, and cyberwarfare with other influencing methods, such as fake news, diplomacy, lawfare, regime change, and foreign electoral intervention. By combining kinetic operations with subversive efforts, the aggressor intends to avoid attribution or retribution. The concept of hybrid warfare has been criticized by a number of academics and practitioners, who say that it is vague and has disputed constitutive elements and historical distortions.

Every age has its own kind of war, its own limiting conditions, and its own peculiar preconceptions.

There is no universally-accepted definition of hybrid warfare; with a debate over its utility and whether it simply refers to irregular methods to counter a conventionally superior force. The vagueness of the term means that it is often used as a catch-all term for all non-linear threats.

Hybrid warfare is warfare which includes some, parts, or all of the following aspects:

The Chief of Staff of the US Army defined a hybrid threat as an adversary that incorporates "diverse and dynamic combinations of conventional, irregular, terrorist and criminal capabilities." The US Joint Forces Command defines a hybrid threat as "any adversary that simultaneously and adaptively employs a tailored mix of conventional, irregular, terrorism and criminal means or activities in the operational battle space. Rather than a single entity, a hybrid threat or challenger may be a combination of state and nonstate actors."

The US Army defined a hybrid threat in 2011 as "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 benefiting effects." NATO uses the term to describe "adversaries with the ability to simultaneously employ conventional and non-conventional means adaptively in pursuit of their objectives."

The former US Army Chief George W. Casey Jr. talked of a new type of war that would become increasingly common in the future: "A hybrid of irregular warfare and conventional warfare." According to the 2017-inaugurated European Centre of Excellence for Countering Hybrid Threats, "hybrid threats are methods and activities that are targeted towards vulnerabilities of the opponent" where the "range of methods and activities is wide".

The concept of grey-zone conflicts or warfare is distinct from the concept of hybrid warfare, although the two are intimately linked, as in the modern era states most often apply unconventional tools and hybrid techniques in the grey-zone. However many of the unconventional tools used by states in the grey-zone such as propaganda campaigns, economic pressure, and the use of non-state entities do not cross over the threshold into formalized state-level aggression.