Ageratum \A*ger"a*tum\ ([.a]*j[e^]r"[.a]*t[u^]m or
   [a^]j`[~e]r*[=a]"t[u^]m), prop. n. [NL., fr. Gr. 'agh`raton a
   sort of plant; 'a priv. + gh`ras old age.] (Bot.)
   A genus of plants, one species of which ({Ageratum  Mexicanum}) has lavender-blue flowers in dense clusters.
   
[1913 Webster]

ageratum
     n 1: rhizomatous plant of central and southeastern United States
          and West Indies having large showy heads of clear blue
          flowers; sometimes placed in genus Eupatorium [syn: {mistflower},
           {mist-flower}, {Conoclinium coelestinum}, {Eupatorium  coelestinum}]
     2: any plant of the genus Ageratum having opposite leaves and
        small heads of blue or white flowers