used stamp; canceled stamp
Refers to postage stamps that have been postmarked or otherwise canceled, often collected by philatelists or donated for charity.
I collect used stamps and donate them to welfare facilities.
The used stamp on this envelope has a very rare postmark.
未使用切手 means 'unused stamp' (mint stamp), the opposite of 使用済み切手.
切手収集 is 'stamp collecting' as a hobby; 使用済み切手 is the specific item collected.
Compound of 使用済み (used, post-use) and 切手 (postage stamp).