A regular private / public key pair, but it's only used to sign a single event, and does not build up reputation, so it's basically a unique one time identity.

For example, your private key signed your profile note (with name, picture, description etc), as well as the note with your reply here, so I know which entity actually responded.

Whereas an ephemeral key is anonymous, I have no clue who liked this post, if it's just one or multiple entities.

Anonymity is beautiful.