Fresnel diffraction describes diffraction when the quadratic propagation wave phase across an aperture must be retained. Its Fresnel propagator becomes the Fraunhofer diffraction description when that curvature is negligible.
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Fresnel diffraction is a type of wave diffraction that occurs when a wavefront encounters an obstacle or aperture with dimensions comparable to its wavelength. It is named after the French engineer and physicist Augustin-Jean Fresnel, who developed the theory to better understand the behavior of light.
Near field approximation to Kirchhoff's diffraction formula, i.e. when the plane of observation is near the object diffracting.