In theoretical physics, cutoff is the maximal or minimal value of energy, momentum, or length, so that the objects with even larger or smaller values than these physical quantities are ignored. It is usually represented within a particular energy or length scale, such as Planck units. An infrared cutoff (longdistance cutoff) is the minimal value of energy  or, equivalently, the maximal wavelength (usually a very large distance)  that we take into account. On the contrary, an ultraviolet cutoff is the maximal allowed energy or the shortest allowed distance (usually a very short length scale). If some quantities are computed as integrals over energy or another physical quantity, these cutoffs determine the limits of integration. The exact physics is reproduced when the appropriate cutoffs are sent to zero or infinity. However, these integrals are often divergent  see IR divergence and UV divergence  and a cutoff is needed. The dependence of physical quantities on the chosen cutoffs (especially the ultraviolet cutoffs) is the main focus of the theory of renormalization group. See also Retrieved from "http://en.wikipedia.org/" 
