The idea that the real-world entities referred to by Information Systems are determinate and uniquely identifiable is a commonly held assumption in the fields of Software Engineering and Database Systems. The concept of identity is also a central topic in Formal Ontology, a discipline that finds application in the field of Information Systems through the use of Foundational Ontologies (FOs). However, while most central concepts of Formal Ontology are, in general explicitly addressed in FOs, the concept of identity has received relatively little attention. The lack of a proper ontological characterization of identity in FOs hinders their application to the analysis of issues related to identification in Information Systems, such as those that arise in conceptual modeling or in database design. This work proposes two distinct, but logically equivalent, formal characterizations of the notion of individual determinacy. Moreover, these characterizations are independent of the particularities of a FO's theory and are defined solely in terms of the structure of the FO's models of portions of reality. Finally, it also introduces a few concepts that are useful in the analysis of identity criteria for the individuals represented using a FO's theory.