Argumentum ad verecundiam
An argument from authority is one in which a proposition is claimed to be true because an esteemed person says it is true. It is a fallacy in that it relies on the person's fame, rather than expertise, or empirical evidence.
- "Albert Einstein believed in God. Are you saying that Einstein was wrong?"
- "Robert Gentry, a world-famous astronomer, calculated that the odds of life appearing by chance are astronomically low."
It is not always a fallacy to say that "So-and-so says that X is true, therefore X is true." For this discussion, it is necessary to distinguish between an expert and an authority.
If a famous astronomer says that the universe is expanding, then it is very likely that the universe really is expanding. If a qualified doctor says that a patient is suffering from Parkinson's disease, that's most likely the case. In these examples, the astronomer and the doctor are experts in a field, and are addressing topics within their area of expertise. As experts, they have studied their respective fields, are familiar with the state of the art, have studied how to recognize certain events, features or conditions, know how to recognize many problems that might lead a layman astray and how to work around them, and so forth. When we take an expert's word for something, we are saying, in effect, that if we had the time to learn as much about the field as the expert does, we would examine the evidence and reach the same conclusion.
Naturally, this applies only to experts speaking within their own field: there is no a priori reason to take an astronomer more seriously than anyone else on the subject of foreign policy or theology.
On the other hand, if the Pope says, ex cathedra, that contraception is a sin, then that's true as well. In this case, the Pope is an authority in matters of sin: it is his job to determine what is and isn't a sin in the Catholic church. In a very real sense, contraception is a sin not because it contradicts the Bible in some way, but merely because the Pope has said so.
Note that in science, there are experts, but no authorities.