A fallacy is an argument that looks persuasive but fails logically. Informal fallacies fail in content — they distract, misrepresent or overreach. Formal fallacies fail in structure — the conclusion does not follow even if the premises are true. Indian logic adds hetvabhasa, defects of the reason (hetu) in an inference.
Worked example: "Either you support the new policy or you are against progress." This is a false dilemma. It presents two options as exhaustive when others exist: neutrality, partial support, support for a different policy, or opposition on grounds unrelated to progress. Once the hidden third option is named, the argument collapses. Spotting the excluded middle is the whole skill.