A postage stamp is a small piece of gummed paper that you buy from the post office and stick on an envelope or package before you post it.
[formal]
postage stamp in British English
noun
1.
a printed paper label with a gummed back for attaching to mail as an official indication that the required postage has been paid
2.
a mark directly printed or embossed on an envelope, postcard, etc, serving the same function
postage stamp in American English
a government stamp to be put on a letter or package as a sign that the postage has been prepaid: it is either a small, printed, gummed label or a design imprinted on an envelope, postal card, etc.