The Method Engineering (ME) community is a prolific research domain where competing Situational Method Engineering (SME) approaches have been defined and used for composing, adapting or/and configuring a method into modular constructs according to their own modularization vision. This diversity shows the richness of the ME domain but implies some drawback like unnecessary confusion for non ME expert, lack of standard & interoperability, lack of implementation tool. However, researchers are agreed that a common ground in SME is a hot matter of discussion. Assuming that the differences between SME approaches are purposeful, we propose to reach a semantic common ground on what types of core concepts constitute a method descriptor. To achieve it, an ontology-based approach is applied in SME to design an ontology of method descriptors as a domain ontology. The semantics of the six most popular SME approaches modular constructs are defined according to this ontology in order to show its usage and its relevance. Finally, usage scenarios have been sketched to show that the ontology can be the start up phase for reducing the ME drawbacks mentioned above.