What's the opposite of "authenticator"?
An authenticator is:
"a person or thing that authenticates."
What do you call the person who is being authenticated? Assuming the use case where a person authenticates another person.
I don't believe there is an official word for that, but I'd have coined "authenticatee" (compare "employee": a person who is employed). Google search reveals that Oracle uses "authenticatee" in its documentation. You could probably get by with this in technical documentation, as I suspect the intent would be understood. Otherwise I'd suggest "authenticated person" or "candidate for authentication" (depending on whether the authentication is complete or pending).