Sociologists frequently invoke the concept of form when analyzing organizations, collective action, art, music, culture and other phenomena. Nonetheless, the form concept has not received careful theoretical analysis, either generally or in specific context. Using the tools of formal logic and set theory, we propose a language for defining social forms that is sufficiently general to incorporate feature‐based, position‐based and boundary‐based approaches to defining forms. We focus on organizational forms although we intend our conceptualization to be general. We define forms as a type of socially coded identity. We define identity in terms of social codes that specify the properties that an entity can legitimately possess. These codes can be enforced by insiders or outsiders. We claim that one knows that a social code exists when one observes that departures from the codes after periods of conformity cause a devaluation of the entity by relevant insiders and/or outsiders. This construction allows us to define a population as the set of entities with a common minimal external identity in a bounded system in a period of time. The minimal property ensures that we localize to the most specific socially enforced identities. The reliance on identities instead of forms allows us to define populations that never achieve form status and to extend population definitions back to the period of early legitimation. Research design implications follow.