Definitions fall into two different categories. In the first, and most common diplomatic usage, multilateralism refers to “the practice of coordinating national policies in groups of three or more states through ad hoc arrangements or by means of institutions”. It is a nominal, or quantitative description referring simply to cooperation among a group of (more than two) actors. Multilateral actions or institutions in this sense, can be clearly contrasted with unilateral, or bilateral, actions. In Keohane's view, multilateral institutions are simply “multilateral arrangements with persistent sets of rules”.

John Ruggie takes issue with this minimalist, nominal definition. He argues that multilateralism is a “generic institutional form” and says that the conventional definition does not tell what makes these institutions multilateral. Ruggie argues that multilateralism is not simply a question of numbers: there have been plenty of arrangements in history involving three or more states that have been, in effect, systemic forms of bilateralism. Therefore, he concludes, there is an essential qualitative dimension which distinguishes multilateralism from these kind of arrangements or international organizations.

Ruggie argues multilateral relationships involve three or more states coming together to tackle a specific issue or set of issues on the basis of specific generalized principles of conduct. These principles specify what constitutes “appropriate conduct for a class of actions” irrespective of the particular interests of the participants or the circumstances that may exist. He argues there are several clearly identifiable qualities that constitute multilateralism. These principles are indivisibility, non-discrimination, and diffuse reciprocity. For example, “it is GATT [General Agreement on Trade and Tariffs] members' adherence to the MFN [most-favourednation] norm [of reciprocated non-discrimination] which makes the system of trade an indivisible whole, not some attribute of trade itself.” All members agree to treat other members in a similar manner. Analogously, for a collective security arrangement to be multilateral, all its member states are required to respond to aggression wherever and whenever it occurs — not simply when it suits their interests.