The argument from authority, or more formally, the argument from false or misleading authority, argumentum ad vericundiam, is a logical fallacy which provides an argument from an authority, but on a topic outside of the particular authority’s expertise or on a topic on which the authority is not disinterested (i.e., is biased).
Almost any subject has an authority on every side of the argument, even where there is generally agreed to be no argument. When correctly applied, it can be a valid and sometimes essential part of an argument that requests judgement or input from a qualified or expert source.
The works (almost always published and peer-reviewed) of authorities, no matter how eminent or influential, is always judged by the quality of their evidence and reasoning, not by their authority alone. Thus, if someone denies evolution and has a Ph.D. in biology from a respected institution, his words are not evidence, only their high quality published works which support their claims.
Dr. Smith, an expert in computer engineering, does not believe in climate change, so his claims are valid and are evidence that climate change does not exist.