What it is
Using one word in two different senses within the same argument.
In the wild
Laws need lawmakers, so the laws of physics need one.
How to answer it
Define the shared word once and hold that definition through the whole argument.
Broken structure
Equivocation
Also called Ambiguity fallacy
In depth
Origin and naming
Glossed in logic as 'calling two different things by the same name.' It is the lexical half of the ambiguity family — the confusion sits inside a single word rather than in grammar. Sustained across a whole syllogism it becomes the medieval fallacy of the ambiguous middle.
More places it shows up
The contract says fees are 'reasonable', and both sides sign — years later a lawsuit reveals they agreed on the word, never the amount.
A manager promises the role is 'flexible'; the hire hears remote-friendly, the manager meant available on weekends.
A bank advertises 'free checking for everyone' — free to open, and free of any charge the bank chooses not to label a fee.
A government report calls the program a 'success'; in its first use it means completed on time, in its second, that it worked.
How to spot it
- Notice when the argument still sounds right until you substitute a dictionary definition for the key word at each use.
- Notice when agreement happens fast — everyone nodded at the same word for different reasons.
- Notice when the contested word carries one sense in the premise and a looser sense in the conclusion.