A hash function is any well-defined procedure or mathematical function that converts a large amount of data into a small datum, usually a single integer. For questions about hashtags as used to label content on social media, use hashtag. For questions about URLs and HTML anchors, use fragment-identifier. For questions about Ruby's hash type, use ruby-hash.
A hash is any well-defined procedure or mathematical function that converts a large, possibly variable-sized amount of data into a small datum, usually a single integer that may serve as an index to an array. The values returned by a hash function are called hash values, hashcodes, hash sums, checksums or simply hashes. A hash-collision occurs when two unequal datums generate the same hash code with a particular hash function. This can have various negative effects and good hash functions minimize the number of collisions.
For data structures that make use of hash functions and hashcodes, see hashtable, hashset, hashmap, dictionary, and associative-array.
A cryptographically strong hash function has two additional features: it is mathematically proven to be irreversible and minimizes collisions. Irreversibility means that the original data cannot be reconstructed from its hash. For questions specifically about cryptographically secure uses of hash functions, use hash combined with the cryptography tag. Contrast with encryption, which must be reversible.
Hash functions are related to (and often confused with) checksums, check digits, fingerprints, randomization functions, and error-correcting codes. Although these concepts overlap to some extent (some hash functions are specifically designed to also serve as checksums), each has its own uses and requirements and is designed and optimized differently.
For questions about hashtags as used to label and navigate content on social media, use hashtag. For questions about URLs and HTML anchors, use fragment-identifier. For questions about Ruby's hash type, use ruby-hash.
