A private key is necessary to decrypt documents encrypted with a public key in PGP.
In asymmetric cryptography, the key which is held only by the user for signing and decryption.
The confidential half of the asymmetric key pair used in public-key cryptography. Unlike the "secret key" used in symmetric-key cryptography — a single key known by both the sender and the receiver — a private key is known only by the recipient. See public-key cryptography and secret-key cryptography.