Business process modelling languages typically enable the representation of business process models by employing (graphical) symbols. These symbols can vary depending upon the verbosity of the language, the modeling paradigm, the focus of the language, and so on. To make explicit the different constructs and rules employed by a specific language as well as bridge the gap across different languages, meta-models have been proposed in literature. These meta-models are a crucial source of knowledge on what state-of-the-art literature considers relevant to describe business processes. Moreover, the rapid growth of techniques and tools that aim at supporting all dimensions of business processes and not only its control flow perspective, as for instance data and organisational aspects, makes even more important to have a clear idea, already at the conceptual level, of the key process constructs. The goal of this work is to provide the first extensive systematic literature review (SLR) of business process metamodels. This SLR aims at answering research questions concerning: (i) the kind of meta-models proposed in literature; (ii) the recurring constructs they contain; (iii) their purposes; and (iv) their evaluations. The SRL was performed manually considering papers automatically retrieved from reference paper repositories as well as proceedings of the main conferences in the Business Process Management research area. Thirty-six papers were selected and evaluated against four research questions. The results indicate the existence of a reasonable body of work conducted in this specific area, but not a full maturity. In particular, while traditional paradigms towards business process modelling, and aspects related to the business process control flow seem to be well present, novel paradigms and aspects related to the organisational, data and goal-oriented aspects of business processes seem to be still under-investigated.