The use of a standard aspheric profile to describe conventional optical elements in lens design programs has advantages such as independent verification of lens system performance by different individuals, ease of interpretation by different manufacturers, etc. A model of diffractive optical element is proposed for consideration as a standard form to have similar benefits. Diffractive elements are characterized here by (1) the equation of the supporting surface (substrate) and (2) the phase function. The phase function itself consists of two terms: (a) the stigmatic phase term and (b) the astigmatic phase term The stigmatic phase term is a rotationally symmetric function and is related to the property of the diffraction element to image free from aberration one axial point onto another, as in holograms, at a specific (reference) wavelength. The astigmatic phase term is an arbitrary function of the space coordinates, it takes several forms: it can impart aspheric deformations (or corrections) to incident wave fronts or it can be used to model different types of gratings. The substrate can be any of the standard surfaces already in use in lens design. The parameters of the phase function are paraxial diffractive power (the counterpart of paraxial curvature), stigmatic foci position index (the counterpart of the conic constant), coefficients of the astigmatic phase function, reference wavelength and diffracted order number.