The Collaborative International Dictionary of English v.0.48:

Costmary \Cost"ma*ry\ (k?st"m?-r?), n. [L. costum an Oriental
   aromatic plant (Gr. ???, cf. Ar. kost, kust) + Maria Mary.
   Cf. Alecost.] (Bot.)
   A garden plant (Chrysanthemum Balsamita) having a strong
   balsamic smell, and nearly allied to tansy. It is used as a
   pot herb and salad plant and in flavoring ale and beer.
   Called also alecost.
   [1913 Webster]

WordNet (r) 3.0 (2006):

costmary
    n 1: tansy-scented Eurasian perennial herb with buttonlike
         yellow flowers; used as potherb or salad green and
         sometimes for potpourri or tea or flavoring; sometimes
         placed in genus Chrysanthemum [syn: costmary, alecost,
         bible leaf, mint geranium, balsam herb, Tanacetum
         balsamita, Chrysanthemum balsamita]
    2: leaves used sparingly (because of bitter overtones) in sauces
       and soups and stuffings