Discussions of diachronic approaches in terminology are scarce or not to the point. The present paper stresses the necessity of defining evolution as it relates to the signifiant or to the signifié, to the term or to a terminology. At the term level two very different phenomena are presented. At the terminology level a simple diachronic model is set up with the aim of defining evolution and depicting quantitative relations between diachronic sets of terms. To complete the discussion, the French nouns in the 1970 and 1985 edition of a data processing dictionary are analysed in order to identify diachronic quantitative and structural differences.