Definition of "countersign"

countersign

noun
plural countersigns
1. (law) A second signature added to a document to affirm the validity of the signature of the first person.
2. The response to a sign or signal.
verb
third-person singular simple present countersigns, present participle countersigning, simple past and past participle countersigned
1. (transitive) To sign on the opposite side of (a document).
2. (transitive, by extension) To add a second signature to a document, affirming the validity of the signature of another person.