Definitions are usually identified with the sentences of the form “X =df Y”. This form is underspecified in various respects; in particular, it is not quite clear what is expressed by =df. The present paper is an attempt to answer the following question: What kind of relation can be represented by =df? It is argued that for every sentence of the form “X =df Y” there is a corresponding sentence in which =df can be replaced by = provided the Y part of the definition is supplemented with further information. This suggestion is made possible by a certain classification of definitions (cf. Zouhar 2014) that is briefly summarized in the present paper as well.