An
ephemeral key is a temporary cryptographic key used for a single session and then discarded. Provides Perfect Forward Secrecy — compromising the long-term key doesn't expose past sessions.
"Ephemeral" in a cipher suite name (DHE, ECDHE) = new key per session = forward secrecy. TLS 1.3 requires ephemeral keys. Long-term keys are only used for authentication, not encryption.