A private key is one of the two kinds of keys used on the Public Key Infrastructure. Not being openly available to anyone, a private key is held by only one person. A message encrypted by the public key can only be decrypted by the private key that corresponds to that public key. As an example, when an individual wants to send a message to someone, the sender's side uses the public key of the receiver's side to encrypt the message; the receiver's side then uses their own private key to decrypt the message.
