Design of collaborative learning (CL) scenarios is a complex task, but necessary if the goal of the collaboration is learning. Creating well-thought-out CL scenarios requires experience and knowledge about different learning/instructional theories and practices related to collaboration. Inexperienced designers, who may not have all the needed knowledge to formulate pedagogically sound collaborative learning plans, may have difficulties to design CL activities. Thus, although CL has shown to be effective, it is not often used in classroom adequately. To tackle this problem, this paper presents an ontology for CL, that aims at modeling not only the learning activities, but also the rationale for their configuration in pedagogical terms. This ontology allows modeling the expected interactions among students as well as the mechanisms that promote or enhance collaboration structured across several sessions and involve individual and collaborative activities. Finally, the paper discusses an application of the presented ontology to support the design of pedagogically sound CL scenarios.