This paper aims to define as rigorously as possible the semantic operation of approximation as one of the strategies applied for identifying a certain referent through a procedure of categorization. For the author, a sound approximation instance must meet the five following semantic conditions: (i) to operate at the propositional level; (ii) and specifically on the description of the referent made by the speaker, (iii) to contain the feature ‹vagueness› in opposition to ‹exactness›; (iv) to be done through an implicit ontological comparison with a reference value and (v) express the gap between the exact and the reference representation. To these semantic conditions two further pragmatic conditions must be added: (vi) the cognitive status of the speaker and (vii) the communicative objective pursued. The final section describes the multiple discursive functions of the approximation operation.