hash function:a mapping of a large data set to a much smaller one that preserves a desired peculiarity of the original data. The input data may be of complex type such as objects, while the output may be a string or a set of integers (or, possibly, just a single integer).
An example of a hash function might be as follows: Suppose a restaurant’s reservations for tonight are five tables of four, ten of two, and a large party of eight. Describing that as reservations for 48, you’ve used a hash function.