Private Key

The secret part of an asymmetric key pair that is typically used to digitally sign or decrypt data.
SOURCE: SP 800-63

A cryptographic key, used with a public key cryptographic algorithm, that is uniquely associated with an entity and is not made public. In an asymmetric (public) cryptosystem, the private key is associated with a public key. Depending on the algorithm, the private key may be used, for example, to:
1) Compute the corresponding public key,
2) Compute a digital signature that may be verified by the corresponding public key,
3) Decrypt keys that were encrypted by the corresponding public key, or
4) Compute a shared secret during a key-agreement transaction.
SOURCE: SP 800-57 Part 1

A cryptographic key used with a public key cryptographic algorithm, which is uniquely associated with an entity, and not made public; it is used to generate a digital signature; this key is mathematically linked with a corresponding public key.
SOURCE: FIPS 196

A cryptographic key, used with a public key cryptographic algorithm, that is uniquely associated with an entity and is not made public.
SOURCE: FIPS 140-2

In an asymmetric cryptography scheme, the private or secret key of a key pair which must be kept confidential and is used to decrypt messages encrypted with the public key or to digitally sign messages, which can then be validated with the public key.
SOURCE: CNSSI-4009