Page History

1. In some cases, "authority" is used to refer to someone or something that is an authority. E.g., one can be an authority in a field of knowledge, or one can be an authority in public life -- these life—these being two different ways of being an authority.

...

6. Thus X is a de facto authority for Y in D by reason (R) of X's knowledge and/or skill if Y believes what X says with respect to D, or holds it to be true with a greater degree of probability, simply because X says it -- orit—or, alternatively, if Y does what X does, or says to do, simply because X does it, or says to do it.

...