In calculus, absolute continuity is a smoothness property of functions that is stronger than continuity and uniform continuity. The notion of absolute continuity allows one to obtain generalizations of the relationship between the two central operations of calculus—differentiation and integration. This relationship is commonly characterized (by the fundamental theorem of calculus) in the framework of Riemann integration, but with absolute continuity it may be formulated in terms of Lebesgue integration. For real-valued functions on the real line, two interrelated notions appear: "absolute continuity of functions" and "absolute continuity of measures." These two notions are generalized in different directions. The usual derivative of a function is related to the "Radon–Nikodym derivative", or "density", of a measure.