Ephemeral key
A key generated for a single session and discarded afterwards, and the basis of forward secrecy.
The word comes from the Greek ephemeros, lasting one day. An ephemeral key is generated for one handshake and destroyed when it ends. If the server's long-term private key leaks later, the attacker still cannot open old sessions, because the session key was never stored anywhere.
DHE and ECDHE do this. Static RSA key exchange in old TLS versions did not, and TLS 1.3 has removed the static variants altogether, which is one of the few times a protocol got stricter without getting more complicated.