Hash functions used in cryptocurrency are covered in full under Cryptographic Hash Function, which explains the security properties that blockchains depend on. This page gives a short definition of the wider term and explains why the two are not quite the same.
A hash function is any function that takes input of any length, such as a word, a file or a block of transactions, and produces an output of fixed length, called a hash or digest. The same input always gives the same output. Ordinary hash functions are used throughout computing for tasks such as quickly looking up records in a table or detecting accidental errors in data, and for those jobs speed matters more than security.
A cryptographic hash function adds stronger guarantees. It should be practically impossible to work back from the output to the input, to find two inputs with the same output, or to predict how a small change in the input will change the result. Those properties let blockchains link blocks together, secure proof of work and create addresses. Bitcoin, for example, relies on SHA-256, and Ethereum on a function from the Keccak family. When crypto glossaries say "hash function", they almost always mean the cryptographic kind, which is why this site covers it under the fuller name. Passwords are also stored as hashes, often with a salt added.