Defining or understanding a product in terms of its functions facilitates a wide variety of tasks such as design synthesis, modeling, and analysis. However, the lack of a semantically correct formal representation of product functions creates a barrier to their effective capture, exchange, and reuse. This paper presents Function Semantics Representation, a rule-based ontological formalism that is consistent with the Semantic Web standards to capture different components of a product function. In particular, the Semantic Web Rule Language is used to overcome limitations in using the basic Web Ontology Language ontology to explicitly capture advanced semantics essential to completely represent product functions. This enables support for an effective reasoning mechanism to develop and validate the product function (or functional model). We present examples that demonstrate consistency checking and the ability to retrieve functionally similar products from a repository.