Private Key

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, that is uniquely associated with an entity and is not made public.
SOURCE: FIPS 140-2