Language can be modelled in various ways, highlighting either its social or systemic character. The author assumes that language is a normative phenomenon enabling speakers to communicate. At any particular time language is used, however, we are capable of determining the function which maps the expressions produced using this language to their meanings. In this contribution the author proposes a functional model of language in a synchronic sense. This model also resolves various complications with ambivalence, etc. Further, he also proposes a model of language in a diachronic sense as a function from possible worlds and time instants to languages in a synchronic sense. Thus the intuitive idea of language as a changing entity is captured. Both models are constructed to be the tools serving mainly the investigation of semantic properties of expressions of that language.