1. <database> A value used to identify a record in a database, derived by applying some fixed function to the record. The key is often simply one of the fields (a column if the database is considered as a table with records being rows, see "key field"). Alternatively the key may be obtained by applying some function, e.g. a hash function, to one or more of the fields. The set of keys for all records forms an index. Multiple indexes may be built for one database depending on how it is to be searched.
2. <cryptography> A value which must be fed into the algorithm used to decode an encrypted message in order to reproduce the original plain text. Some encryption schemes use the same (secret) key to encrypt and decrypt a message, but public key encryption uses a "private" (secret) key and a "public" key which is known by all parties.