A metrological approach to calibration, from the analytical chemistry point of view, is discussed. The terms referring to calibration, and usually employed in the literature (such as calibration point, correction, calibration factor, calibration curve and calibration function) are explained and clarified. Various definitions of calibration, compiled from several international standards, are placed in a metrological context. Likewise, the different types of calibration, which are not always well established, such as direct and indirect calibration, equipment and process calibration are described. Recovery factors and uncertainty in the calibration are also considered.