Modularity in biological systems is a central theme in current research. However, there are some problems related to the vague definitions of the concept of a module that impede both its empirical application and the comparison of data from different studies. In this article, we propose to characterize this concept which is consistent with the diverse ways in which it is conceptualized in the specialist literature. We test our characterization by analyzing the case study of the craniofacial complex. We conclude that the proposed delimitation allows a better understanding of modules since it differentiates between the concept of the module as a state (morphofunctional modules) and as a process (a product of ontogenetic or phylogenetic history).