What does "tessera" mean?

Tessera: A small piece of marble, glass, earthenware, or the like, having a square, or nearly square, face, used by the ancients for mosaic, as for making pavements, for ornamenting walls, and like purposes; also, a similar piece of ivory, bone, wood, etc., used as a ticket of admission to theaters, or as a certificate for successful gladiators, and as a token for various other purposes. Fairholt.