The Free On-line Dictionary of Computing (30 December 2018):
A hash codingfunction 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.