Abstract—The design of integrated circuits involves the consideration of a large number of constraints of various types. In addition to the definition of these constraints in a constraint-driven design flow, the declaration of new, yet unknown constraint types might be necessary. We define an ontology for constraints as a universal approach for the definition of constraint types and their behavior. This formal constraint representation categorizes the behavior of constraint types that are common in custom IC design. It also gives special attention to the parameters they constrain, the sensitivity to other parameters and the set of valid target design objects of these types.