(1) A column or an ordered collection of columns that is identified in the description of a table, index, or referential constraint. The same column can be part of more than one key.
(2) A cryptographic mathematical value that is used to digitally sign, verify, encrypt, or decrypt a message. See also private key, public key.
(3) One or more characters within an item of data that are used to uniquely identify a record and establish its order with respect to other records. See also alternate-index record.
(4) A button on a keyboard or key pad.
(5) Information that characterizes and uniquely identifies the real-world entity that is being tracked by a monitoring context.