countersign
noun
Definitions
A second signature added to a document to affirm the validity of the signature of the first person.
The response to a sign or signal.
Example: Only those who give the countersign may enter the Masonic Lodge.
verb
Definitions
To sign on the opposite side of (a document).
(by extension) To add a second signature to a document, affirming the validity of the signature of another person.