When not speaking of arbitrary hash functions, Hash refers to two rounds of SHA-256 . That is, compute a SHA-256 hash of the data followed by another SHA-256 of that hash. This process is used in block header hashing, transaction hashing, creating a merkle tree of transactions, or computing a checksum of an address . It is known as BTCHash256() in CoreBitcoin, Hash() in BitcoinQT. It is also available is scripts as OP_HASH256.
Malone, J.A (2015). Glossary of Bitcoin Terms and Definitions. United States: Lulu Press, Inc