Cryptographic Keys

A value used to control cryptographic operations, such as decryption, encryption, signature generation, or signature verification.
SOURCE: SP 800-63

A binary string used as a secret parameter by a cryptographic algorithm.
SOURCE: SP 800-108
 

A parameter used in conjunction with a cryptographic algorithm that determines the specific operation of that algorithm.
SOURCE: FIPS 201; FIPS 198

A parameter used in conjunction with a cryptographic algorithm that determines
1) the transformation of plaintext data into ciphertext data,
2) the transformation of ciphertext data into plaintext data,
3) a digital signature computed from data,
4) the verification of a digital signature computed from data,
5) an authentication code computed from data, or
6) an exchange agreement of a shared secret.
SOURCE: FIPS 140-2