The Collaborative International Dictionary of English v.0.48:

Plaque \Plaque\, n. [F. Cf. Plack, and see Placard.]
   Any flat, thin piece of metal, clay, ivory, or the like, used
   for ornament, or for painting pictures upon, as a slab,
   plate, dish, or the like, hung upon a wall; also, a smaller
   decoration worn on the person, as a brooch.
   [1913 Webster]

WordNet (r) 3.0 (2006):

plaque
    n 1: (pathology) a small abnormal patch on or inside the body
    2: a memorial made of brass [syn: brass, memorial tablet,
       plaque]

Moby Thesaurus II by Grady Ward, 1.0:

66 Moby Thesaurus words for "plaque":
   arch, award, badge, barrow, boundary stone, brass, brooch, bust,
   cairn, cenotaph, column, cromlech, cross, cup, cyclolith, dolmen,
   footstone, grave, gravestone, headstone, hoarstone, honor,
   inscription, insignia, marker, mausoleum, medal, medallion,
   megalith, memento, memorial, memorial arch, memorial column,
   memorial statue, memorial stone, menhir, monolith, monument, mound,
   necrology, obelisk, obituary, panel, patch, pillar, pin, plaquette,
   plate, prize, pyramid, reliquary, remembrance, ribbon,
   rostral column, shaft, shrine, slab, stela, stone, stupa, tablet,
   testimonial, tomb, tombstone, tope, trophy