A data structure used to quickly make computations or retrievals of certain values. In image processing, it is used to store precalculated values of an equation instead of calculating the value each time it is requested. For example, if the equation is y = 2x, and you know that there are at most n distinct values, then you can create an array of size n (one for each input) that stores the precalculated value of the equation for the corresponding input (for example,[1] = 0, [2] = 2, [3] = 4, ... [4] = 2n ).