Definition
A false dilemma — also called a false dichotomy or either/or fallacy — is an informal fallacy in which an argument restricts the available options to two when in fact a wider range of choices, intermediate positions, or combinations is open. The classic schematic form is: "Either A or B; not A; therefore B" — valid as a piece of disjunctive reasoning, but unsound whenever the opening disjunction is itself incomplete.
The fallacy is rhetorical rather than purely formal. It exploits the cognitive comfort of a clean binary to suppress the alternatives a fuller survey would surface.
Why it matters
How it works
A false dilemma sneaks an unstated premise into an otherwise valid argument. The valid skeleton is disjunctive syllogism: from "A or B" and "not A", infer "B". The argument is deductively sound only if "A or B" actually covers every relevant possibility. False dilemmas slip in a disjunction that omits options C, D, and the various mixtures of partial-A-and-partial-B. The conclusion then follows from the rules of inference, but it follows from a smuggled falsehood about the world.
The fallacy is hard to see because the binary frame is offered as background, not as a contested claim. The audience accepts "A or B" implicitly and then debates which side of it to occupy — never noticing that the most defensible position may sit outside both. Defusing it is a single move: question the disjunction itself. Ask what other options exist, or whether the two named options can be combined. The rhetorical force vanishes the moment the missing alternatives are made visible.