The fallacies of ambiguity slide between two meanings. The simplest one rides on a single ambiguous word.
Key idea
Equivocation uses a word in one sense in one premise and in another sense elsewhere, so the argument only seems to connect.
Why it works
An argument links its premises through shared words. If the word changes meaning, the link is broken, even though the spelling hides the break. With the meanings written out, the premises are about two different things.
Worked example: Swap in the meanings
"Ben is a big fan of the town team. A fan is a machine that blows air. So Ben is a machine that blows air."
- Line 1: "fan" means a supporter.
- Line 2: "fan" means a machine.
- Replace each with its meaning: the premises no longer share a term.
Answer: Equivocation.
Worked example: Hidden in "nothing"
"Nothing is better than a long vacation. A cold sandwich is better than nothing. So a cold sandwich is better than a long vacation."
- First "nothing": no other thing (no thing beats a vacation).
- Second "nothing": having nothing at all.
- Two senses, so the chain breaks.
Answer: Equivocation.
Common mistake
Missing the shift because the spelling stays the same. Replace the word with what it means in each line; if the two meanings differ, the gap shows at once.