Forgery used as a verb means the act of forging or producing a copy of a document, signature, banknote, or work of art with the intention of creating the impression the object is authentic.
"When they realize that the copy was not authentic, he was accused of forgery."
Forgery used as a noun means a forged or copied document, singature, banknote or work of art.
"The hundred dollar bill was a forgery."