The Collaborative International Dictionary of English v.0.48:

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]

WordNet (r) 3.0 (2006):

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, ageratum, Conoclinium
         coelestinum, Eupatorium coelestinum]
    2: any plant of the genus Ageratum having opposite leaves and
       small heads of blue or white flowers