Ageratum

(noun) rhizomatous plant of central and southeastern United States and West Indies having large showy heads of clear blue flowers; sometimes placed in genus Eupatorium. Synonyms: mistflower, mist-flower, ageratum, Conoclinium coelestinum, Eupatorium coelestinum.

(noun) any plant of the genus Ageratum having opposite leaves and small heads of blue or white flowers. Synonyms: ageratum.