谬论或（思考）谬误是指不当的推理言论或推理思路。恶意或别有所图的谬论又称作诡辩。

知识或数据的错误，称作讹误，但日常习惯上，谬论或谬误有时也用于泛指错误的言论或想法。

一个好的论证必须有效且健全。所谓有效是指如果前提为真则结论必定为真，所谓健全是指除了有效以外，所有前提都是真的，健全的论证保证结论为真。反之，如果前提不为真或前提为真不能保证结论为真，则不是好论证。

论证有谬误，不表示其结论不真确（参见谬误谬误）；然而，对于尚未相信结论的人而言，论证过程有谬误会导致他们无法被论证说服。

A fallacy is the use of invalid or otherwise faulty reasoning, or "wrong moves" in the construction of an argument. A fallacious argument may be deceptive by appearing to be better than it really is. Some fallacies are committed intentionally to manipulate or persuade by deception, while others are committed unintentionally due to carelessness or ignorance. Lawyers acknowledge that the extent to which an argument is sound or unsound depends on the context in which the argument is made.