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., for 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. 

See NIST SP 800-57 Part 1 Rev. 4 for more information.