The Collaborative International Dictionary of English v.0.48:
countersignature \countersignature\ n. a second confirming signature endorsing a document already signed. Syn: countersign. [WordNet 1.5]WordNet (r) 3.0 (2006):
countersignature n 1: a second confirming signature endorsing a document already signed [syn: countersignature, countersign]