Definition of countersigned:
(n) :
(law) A second signature added to a document to affirm the validity of the signature of the first person.
(v) :
(transitive, by extension) To add a second signature to a document, affirming the validity of the signature of another person.
(v) :
(transitive) To sign on the opposite side of (a document).
(n) :
The response to a sign or signal.