Materialization is a powerful and ubiquitous abstraction pattern for conceptual modeling. Intuitively, it relates a class of categories (e.g., models of cars) and a class of more concrete objects (e.g., individual cars). This paper formalizes the semantics of materialization using the metaclass approach of the TELOS data model. Formulas can be uniformly attached to classes, metaclasses, and meta-attributes to enforce integrity constraints and deductive rules relevant to materialization semantics. The paper also proposes some suggestions for extending TELOS to capture some materialization semantics which cannot be represented with the available constructs.