Nowadays many software development frameworks implement Behavior-Driven Development (BDD) as a mean of automating the test of interactive systems under construction. Automated testing helps to simulate user's actions on the User Interface and therefore check if the system behaves properly and in accordance to scenarios that describe functional requirements. However, tools supporting BDD run tests on implemented User Interfaces and are a suitable alternative for assessing functional requirements in later phases of the development process. However, even when BDD tests can be written in early phases of the development process they hardly can be used with specifications of User Interfaces such as prototypes. To address this problem, this paper proposes to raise the abstraction level of both system interactive behaviors and User Interfaces by the means of a formal ontology that is aimed at supporting test automation using BDD. The paper presents an ontology and an ontology-based approach for automating the test of functional requirements of interactive systems. We demonstrate the feasibility of this ontology-based approach to assess functional requirements in prototypes and full-fledge applications through an illustrative case study of e-commerce applications for buying flight tickets.