Session keys are keys that are generated to be used in a single communication session. Session keys are frequently changed and are discarded when they are no longer needed. For example, TLS uses a separate session key for each connection and S/MIME uses a separate session key for each email message. Typically a symmetric key is used as the session key.
Session keys are volatile. Applications can save these keys for later use or for transmission to other users. For more information, see Cryptographic Key Storage and Exchange.