The Collaborative International Dictionary of English v.0.48:

Tessera \Tes"se*ra\, n.; pl. Tesserae. [L., a square piece, a
   die. See Tessellar.]
   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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

tessera
    n 1: a small square tile of stone or glass used in making
         mosaics

Moby Thesaurus II by Grady Ward, 1.0:

47 Moby Thesaurus words for "tessera":
   ID card, badge, calligram, calling card, card, check, checker,
   checkerboard, checkerwork, chessboard, colors in patches,
   countermark, countersign, crazy-work, credentials, dog tag,
   harlequin, identification, identification badge,
   identification tag, initials, inlay, insert, insertion, inset,
   letter of introduction, marquetry, monogram, mosaic, open sesame,
   parquet, parquetry, password, patchwork, plaid, press card,
   secret grip, serial number, shibboleth, signature, tartan,
   tessellation, tesserae, token, variegated pattern, visiting card,
   watchword