May refer to an ECDSA public or private key, or AES symmetric encryption key. AES is not used in the Bitcoin protocol itself, but is used only to encrypt the ECDSA keys and other sensitive data, so “key” usually refers to an ECDSA key. Usually refers to a private key as a public key can always be derived from a private key.
Malone, J.A (2015). Glossary of Bitcoin Terms and Definitions. United States: Lulu Press, Inc