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).
Sources:
NIST SP 800-57 Part 2 Rev.1
A cryptographic key that is generated for each execution of a cryptographic process (e.g., key establishment) and that meets other requirements of the key type (e.g., unique to each message or session).
Sources:
NIST SP 800-57 Part 1 Rev. 5
under Ephemeral key