Source : Webster's Revised Unabridged Dictionary (1913)

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}.

Source : WordNet®

Chrysanthemum balsamita
     n : 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}]