Source : Webster's Revised Unabridged Dictionary (1913)

Ageratum \A*ger"a*tum\, n. [NL., fr. Gr. ? a sort of plant; 'a
   priv. + ? old age.] (Bot.)
   A genus of plants, one species of which ({A. Mexicanum}) has
   lavender-blue flowers in dense clusters.

Source : WordNet®

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