The Free On-line Dictionary of Computing (30 December 2018):

hash function

    A hash coding function which assigns a data
   item distinguished by some "key" into one of a number of
   possible "hash buckets" in a hash table.  The hash function is
   usually combined with another more precise function.

   For example a program might take a string of letters and put
   it in one of twenty six lists depending on its first letter.
   Ideally, a hash function should distribute items evenly
   between the buckets to reduce the number of hash collisions.
   If, for example, the strings were names beginning with "Mr.",
   "Miss" or "Mrs." then taking the first letter would be a very
   poor hash function because all names would hash the same.

   (1997-08-03)