An ontological model can be used to define a rich semantic landscape of terminology for a technical domain. The advantages of doing so are a high degree of self-consistency between the domain’s technical language terms, which increases the quality of their natural language usage. This paper explores an initial attempt to construct an ontological model of data safety terminology for the Data Safety Initiative project. In doing so it introduces the systematic approach ontological modelling brings to not only defining but also understanding a domain’s terminology.