There is no single definition of what constitutes a middle power. Adam Chapnik argues that “for all its importance, [the term] ‘middle power’ is rarely defined and limited explanations are never specific.” According to a major work on the subject, there are at least four distinct approaches to defining a middle power. In practice, the term most commonly refers to a state on the basis of its position in an informal international hierarchy. In this view, middle powers occupy the “middle” point on one of several gauges of national power. These include size, geostrategic location, population, gross national product (GNP), military capability, and so on. While measuring precisely what is the “middle” and what is not can be highly problematic; the concept satisfies “the intuitive desire to differentiate between those states which clearly are not great powers but are not minor powers either”.

A second definition is based on conceptual position. In this view, middle powers are just that — in the middle between major powers. This approach has two variants: one, with obvious Cold War baggage, suggests middle powers are in the middle, between ideologically polarized great powers. An alternative geographic definition argues that middle powers are states that are powerful within their own particular geographic region. A third definition is a normative one. This asserts that middle powers are “potentially wiser or more virtuous” than either great powers or small states. Middle powers are viewed as somehow more trustworthy members of the international community because they can exert diplomatic influence without resorting to the use of force.

Recognizing the limitations of all these approaches, an alternative behavioural formulation was put forward by Andrew Cooper, Richard Higgott, and Kim Nossal in their work, Relocating Middle Powers. Rather than focus on the traditional criteria, they identify middle powers on the basis of the diplomatic behaviour they display in common.