private key

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun One of two keys used in public key cryptography and held privately by the recipient of encrypted data, typically used to sign digital signatures and to decrypt data that was encoded using the recipient's public key.
• noun A key that is used both to encrypt and decrypt data in private key cryptography.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The unpublished key in a cryptographic system that uses two keys.