countersign

Etymology

From Middle French contresigne. Equivalent to counter- + sign.

noun

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.
   Only those who give the countersign may enter the Masonic Lodge.

verb

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.