Hashing is an example of a space-time tradeoff. Many hash table designs also allow arbitrary insertions and deletions of key–value pairs, at amortized constant average cost per operation.

In a well-dimensioned hash table, the average time complexity for each lookup is independent of the number of elements stored in the table. Such collisions are typically accommodated in some way. Ideally, the hash function will assign each key to a unique bucket, but most hash table designs employ an imperfect hash function, which might cause hash collisions where the hash function generates the same index for more than one key. During lookup, the key is hashed and the resulting hash indicates where the corresponding value is stored. A hash table uses a hash function to compute an index, also called a hash code, into an array of buckets or slots, from which the desired value can be found. In computing, a hash table, also known as a hash map, is a data structure that implements an associative array, also called a dictionary, which is an abstract data type that maps keys to values. For the South Park episode, see Rehash (South Park).

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