This sort of argument attempts to assert a claim's truth, calling on supposed--often misleading or irrelevant, sometimes even false--qualifications, virtues, and certifications of the one making the claim to 'prove' the claim true, despite informal logic and real evidence.
This argument in both valid and fallacious usage usually has the following layout: A has apparent or claimed qualifications Q. A says that X is true. So X is true.
The valid form of this argument, when qualifications are both real and relevant, attaches the qualifier 'probably' to the alleged truth of claim X, since in this case the truth of the claim cannot follow necessarily or be known with certainty.
Or to put it another way...
Dr. Von Blümrich is a great rocket scientist. Dr. Von Blümrich claims that the vision described in the Biblical book of Ezekiel was that of a visitation by ancient astronauts in a rocket-powered spacecraft. Therefore, despite a complete lack of any physical evidence of a spacecraft landing in the Middle East at around that time, it must be true that Ezekiel's vision was literally a physical event, and described an alien rocketship, not Ezekiel hallucinating out of his tree in a mystical experience.
People, I sh*t you not. Someone actually used that argument on me, and it wasn't convincing then either...
Another example of this style of argument, used on me by someone who otherwise has the intellectual resources to know better than to commit such an obvious fallacy, is...
"Time travel is impossible, because Professor so-and-so, at such-and-such University, whom I highly respect because he's very intelligent, said that it is..."Ahem...
There is a wide variety of supposed but false or irrelevant virtues invoked in this form of specious argument--itself a subset of genetic fallacy, an argument that uses the origin of a claim to assert its truth or falsehood, including such things as wealth, sincerity, intelligence, unconventionality, age (or youth), ancient wisdom (Appeal to Antiquity), wide social acceptance (Appeal to Popularity), celebrity (Appeal to Celebrity), newness (Appeal to Novelty), beauty, strength or power, social status, subjective personal experience, quotations by someone famous taken out of context or even fabricated (Quote-Mining), purity, virginity, charity, sincerity, claims of impending acceptance (a combination of Argument from Authority and Unstated Premise), piety, self-assumed but unsubstantiated credentials, claimed divine inspiration or origin, vague references to 'experts,' 'scientists,' 'researchers,' or other authorities that cannot be followed up on, and even such normally non-advantageous things as poverty and persecution.
The list goes on, and some of these may even shade into other logical fallacies.
This fallacy attempts to deceive about the nature of the evidence it presents, a gambit to disguise itself as valid logic and actual evidence while not really presenting either.
An acceptable form of argument even in it's fallacious form in medieval scholasticism, we've moved on a bit since then, and in that usage it's no longer broadly accepted by philosophers and logicians as sound or even informally valid reasoning.
The Argument from Authority is always fallacious when the authority so name-dropped is considered in effect to be incontrovertibly correct, its not-so-evil mirror universe twin, an Argument by Authority, as noted above, can be a valid form of argument.
Finally, as mentioned above, this can shade into an ad Hominem, particularly a Positive ad Hominem, with a fuzzy but real division between them in some arguments, in that often those people in the best position to examine the truth or falsehood of a statement just happen to be those individuals with experience, a vested interest and personal involvement in the subject at hand.