A revenue stamp, tax stamp or fiscal stamp is a (usually) adhesive label used to collect tax

Tax

To tax is to impose a financial charge or other levy upon a taxpayer by a state or the functional equivalent of a state such that failure to pay is punishable by law. Taxes are also imposed by many subnational entities...

es or fees on documents, tobacco, alcoholic drinks, drugs and medicines, playing cards, hunting licenses, firearm registration, and many other things. Typically businesses purchase the stamps from the government, and attach them to taxed items as part of putting the items on sale, or in the case of documents, as part of filling out the form.

Revenue stamps often look very similar to postage stamp

Postage stamp

A postage stamp is a small piece of paper that is purchased and displayed on an item of mail as evidence of payment of postage. Typically, stamps are made from special paper, with a national designation and denomination on the face, and a gum adhesive on the reverse side...

s, and in some countries and time periods it has been possible to use postage stamps for revenue purposes.
Revenue stamps are stamps used to collect taxes and fees.