A symmetric key is a secret key shared between parties and used for both encryption and decryption.
Symmetric cryptography uses the same key to encrypt and decrypt. It is efficient for bulk data encryption but requires a secure way to distribute and rotate keys.
A cryptographic key that is used to perform both the cryptographic operation and its inverse, for example to encrypt plaintext and decrypt ciphertext, or create a message authentication code and to verify the code.
Also, a cryptographic algorithm that uses a single key (i.e., a secret key) for both encryption of plaintext and decryption of ciphertext.