1. countermark - Noun
2. countermark - Verb
A mark or token added to those already existing, in order to afford security or proof; as, an additional or special mark put upon a package of goods belonging to several persons, that it may not be opened except in the presence of all; a mark added to that of an artificer of gold or silver work by the Goldsmiths' Company of London, to attest the standard quality of the gold or silver; a mark added to an ancient coin or medal, to show either its change of value or that it was taken from an enemy.
An artificial cavity made in the teeth of horses that have outgrown their natural mark, to disguise their age.
To apply a countermark to; as, to countermark silverware; to countermark a horse's teeth.
Source: Webster's dictionary