Label or coupon or tab in philately is a part of sheet of stamps separated from them with perforation (or narrow white margin in imperforate stamps). It cannot be used for postage because it does not have face value and any indication of a postal administration that issued such stamps with labels. The notion of label should not be messed up with the term "gutter" or with a margin of a stamp sheet.
Sometimes, label is also a stamp-like adhesive of no postal value, often used for promotional purposes.
The Soviet Union 1970 CPA 3868 stamp with label (Friendship Tree, Sochi with label).jpg
Stamp of the Soviet Union with a label dedicated to the Tree of Friendship in Sochi (1970)
Stamp US 1966 5c Cassatt with Zippy.jpg
Mr. ZIP on a stamp sheet margin (not a label!). An US postage stamp (1966) featuring "The Boating Party" painted by Mary Cassatt in 1893–1894