In PublicKeyEncryption, a Signature is a CryptographicHash of a message which is signed with the PrivateKey and appended to the message. This allows any holders of the PublicKey to verify that the signature was created by a holder of the private key. The exact meaning of the signature depends on the system (and often what is being signed), for example signing someone else's PublicKey after a KeySigningParty is quite different to signing an Email to a friend.
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