The Appeal to Authority

Also known as argumentum ab auctoritate)

This is a common fallacy which attempts to establish an inductive argument from a generalisation or a cherry picked example. It takes this form:

A is an authority on a particular topic
A says something about that topic
A is probably correct

The argument is fallacious because, although authorities can be correct in judgments related to their area of expertise more often than laypersons, they can still come to the wrong judgments. Therefore, the appeal to authority is not a generally reliable argument for establishing facts.

It is often used by referring to an expert whilst ignoring the fact that other experts disagree.  

A variation is the Appeal to Anonymous Authorities where the authority is not mentioned (for example "scientists say...")

Example:
Scientists agree that the universe is finely tuned.

Explanation:
Which scientists? What about the scientists that don't agree? In what sense are these scientists using the term "finely tuned"? How can a scientist be an authority on "fine tuning" when it is not a scientific concept? And so on.