NIST SP 800-56A
Ephemeral Key - A key that is intended for a very short period of use. The key is ordinarily used in exactly one transaction of a cryptographic scheme; an exception to this is when the ephemeral key is used in multiple transactions for a key transport broadcast. Contrast with static key.
NIST SP 800-57P1
Ephemeral Key - A cryptographic key that is generated for each execution of a key establishment process and that meets other requirements of the key type (e.g., unique to each message or session). In some cases ephemeral keys are used more than once, within a single "session (e.g., broadcast applications) where the sender generates only one ephemeral key pair per message and the private key is combined separately with each recipient's public key.