informal fallacy
A reasoning mistake found in what an argument says, not in its form.
An informal fallacy is a defect in an argument's content: its premises are irrelevant to the conclusion, too weak to support it, assume what is in question, or depend on a shift in meaning. Because the defect is not in the form, an informally fallacious argument can still be valid.
Examples
"Everyone I know loves this phone, so it must be the best phone" is an informal fallacy (appeal to popularity).
"I always tell the truth, so I am telling the truth now" is valid in form but begs the question.
See also