Symmetric encryption/decryption key

Definition
In a symmetric encryption/decryption key,

"[a] symmetric key is used to encrypt and decrypt data or messages. For data-in-transit, a symmetric encryption/decryption key may have a short life, typically for each message (e.g., S/MIME message) or for each session (for example a TLS session). For stored data, the symmetric life of the encryption/decryption key tends to be as long as the confidentiality of the data needs to be protected."