nonce

That will do for the nonce, but we'll need a better answer for the long term.

I had thought that the term was a nonce, but it seems as if it's been picked up by other authors.

The protocol opens with A communicating in clear to AS his own claimed identity and the identity of the desired correspondent, B, together with A's nonce identifier for this transaction, I_(A1). ("Nonce" means "used only once.")