The knowledge capture to elaborate domain modeling can have a semasiological approach from a linguistic perspective (from the term to the concept) or the onomasiological approach of an extralinguistic perspective (from the concept to the term). In this paper we expose the methodology supported by the onomasiological approach elaborated by I. Dahlberg, and it demonstrates the roles that the definitions have in this activity. The definition in this case is understood as the product of an agreement established between the modeler/ontologist and the domain expert; It reflects the intended model of representation of the modeled world.