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.
See postage stamp in American Heritage Dictionary 4
noun
A small, usually adhesive label issued by a government and sold in various denominations to be affixed to items of mail as evidence of the payment of postage.
adjective
Very small: postage-stamp villages; a postage-stamp farm.