A fallacy is a defect in reasoning, and fallacies divide into two categories based on where the defect lies: a formal fallacy is a defect in an argument's logical form (the argument's structure is invalid regardless of what content is substituted into it), while an informal fallacy is a defect arising from the content or meaning of the argument's terms, even when the argument's surface form appears valid. This distinction belongs to the domain of critical thinking / informal logic, and it depends on the prior concept of argument validity (whether a conclusion necessarily follows from its premises) as the baseline against which both fallacy types are defined.
A fallacy is a defect in reasoning, and fallacies divide into two categories based on where the defect lies: a formal fallacy is a defect in an argument's logical form (the argument's structure is in…