What characteristic defines symmetric encryption in terms of key usage?

Symmetric encryption is characterized by the use of the same key for both the encryption and decryption processes. This means that the same secret key must be shared and kept secure by all parties involved in the communication. When one party encrypts a message with the symmetric key, the recipient uses that same key to decrypt the message back into its original form. This method relies on the confidentiality of the key, as anyone possessing the key can decrypt the data.

In contrast, asymmetric encryption, where different keys are used (a public key for encryption and a private key for decryption), does not share this characteristic. The absence of keys in some encryption methods does not apply to symmetric encryption, as the very nature of symmetric encryption hinges upon the use and management of keys.