A fallacy is an argument that seems to be better than it really is.
The danger of a fallacy is not that it is obviously wrong. It is that it is not obviously wrong, it wears the shape of a real argument closely enough to pass unnoticed.
Aristotle’s Sophistical Refutations, the closing treatise of his Organon, is the founding taxonomy: arguments “which appear to be refutations but are really fallacies and not refutations.” He divided them into linguistic fallacies, which trade on the ambiguity of words themselves (equivocation, amphiboly, composition, division), and non-linguistic ones, which fail for other reasons (begging the question, false cause, faulty generalisation among them). This thirteen-part taxonomy stood as the field’s reference point for over two thousand years.
The modern study of fallacies was effectively refounded by C.L. Hamblin’s 1970 book Fallacies, which opened by attacking the “debased, worn-out and dogmatic” treatment fallacies received in the logic textbooks of his era, disconnected from the rest of modern logic. That critique is credited with reviving fallacy theory and launching the broader field of informal logic. The stakes Hamblin and Aristotle both point to are practical: knowing the common shapes fallacious reasoning takes is what lets a reader catch an argument that only sounds sound.
Source: Aristotle, Sophistical Refutations; C.L. Hamblin, Fallacies (1970).