A military stamp, is a 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...

 used by a military organisation, in time of war, or while ensuring a peace keeping operation. Often the letters will be transported by the army itself until they reach the country of destination. These stamps were widely used during World War II

World War II

World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

by soldiers wishing to send mail home to their families. The usage of these stamps has been somewhat phased out by the appearance of electronic means of communication.