revenue stamp

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A stamp affixed to an item as proof that a government tax has been paid.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A stamp to affix as proof that the tax due on the marked product has been paid

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a small piece of adhesive paper that is put on an object to show that a government tax has been paid