A person who signs a document or agreement jointly with one or more other people, sharing responsibility for it.
Combines co- (with/together) and signatory (one who signs), from Latin signum (sign). This legal term emerged in the 18th century as commerce grew and partnerships required multiple parties to verify contracts.
The word appears frequently in international treaties—when countries are cosignatories to a climate agreement, they're all equally bound by its terms, making enforcement tricky but cooperation binding.
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