Argument from authority, also authoritative argument and appeal to authority, is a common form of argument which leads to a logical fallacy when misused.
In informal reasoning, the appeal to authority is a form of argument attempting to establish a statistical syllogism. The appeal to authority relies on an argument of the form:
A is an authority on a particular topic
A says something about that topic
A is probably correct
Fallacious examples of using the appeal include any appeal to authority used in the context of logical reasoning, and appealing to the position of an authority or authorities to dismiss evidence, as authorities can come to the wrong judgments through error, bias, dishonesty, or falling prey to groupthink. Thus, the appeal to authority is not a generally reliable argument for establishing facts.
The argument from authority can take several forms. As a syllogism, the argument has the following basic structure:
A says P about subject matter S.
A should be trusted about subject matter S.
Therefore, P is correct.
The second premise is not accepted as valid, as it amounts to an unfounded assertion that leads to circular reasoning able to define person or group A into inerrancy on any subject matter.
Dismissal of evidence
The equally fallacious counter-argument from authority takes the form:
B has provided evidence for position T.
A says position T is incorrect.
Therefore, B's evidence is false.
This form is fallacious as it does not actually refute the evidence given by B, merely notes that there is disagreement with it. This form is especially unsound when there is no indication that A is aware of the evidence given by B.
Appeal to non-authorities
Fallacious arguments from authority often are the result of citing a non-authority as an authority. First, when the inference refers to an inexpert authority, it is an appeal to inappropriate authority, which occurs when an inference relies upon a person or a group without relevant expertise or knowledge of the subject matter under discussion.
However, it is a fallacious ad hominem argument to argue that a person presenting statements lacks authority and thus their arguments do not need to be considered. As an appeal to a perceived lack of authority, it is fallacious for much the same reasons as an appeal to authority.
Use in logic
It is fallacious to use any appeal to authority in the context of logical reasoning. Because the argument from authority is not a logical argument in that it does not argue something's negation or affirmation constitutes a contradiction, it is fallacious to assert that the conclusion must be true. Such a determinative assertion is a logical non sequitur as the conclusion does not follow unconditionally, in the sense of being logically necessary.
The only exceptions to this would be an authority which is logically required to always be correct, such as an omniscient being that does not lie.
Inaccurate Chromosome Number
In 1923, leading American zoologist Theophilus Painter declared based on his findings that humans had 24 pairs of chromosomes. From the 1920s to the 1950s, this continued to be held based on Painter's authority, despite subsequent counts totaling the correct number of 23. Even textbooks with photos clearly showing 23 pairs incorrectly declared the number to be 24 based on the authority of the then-consensus of 24 pairs.
As Robert Matthews said of the event, "Scientists had preferred to bow to authority rather than believe the evidence of their own eyes". As such, their reasoning was an appeal to authority. 
The Tongue Map
Another example is that of the tongue map, which purported to show different areas of taste on the tongue. While it originated from a misreading of the original text, it got taken up in textbooks and the scientific literature  for nearly a century, and remained even after being shown to be wrong in the 1970s and despite being easily disproven on one's own tongue.
An integral part of the appeal to authority is the cognitive bias known as the Asch effect. In repeated and modified instances of the Asch conformity experiments, it was found that high-status individuals create a stronger likelihood of a subject agreeing with an obviously false conclusion, despite the subject normally being able to clearly see that the answer was incorrect.
Further, humans have been shown to feel strong emotional pressure to conform to authorities and majority positions. A repeat of the experiments by another group of researchers found that "Participants reported considerable distress under the group pressure", with 59% conforming at least once and agreeing with the clearly incorrect answer, whereas the incorrect answer was much more rarely given when no such pressures were present.