ageratum

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of various tropical American plants of the genus Ageratum in the composite family, especially A. houstonianum, cultivated for their fluffy blue, purple, pink, or white flower heads.
• noun Any of several other plants having flower clusters similar to those of ageratum.

from The Century Dictionary.

• noun A genus of plants, natural order Compositæ, all American and chiefly tropical, nearly allied to Eupatorium.

from the GNU version of the Collaborative International Dictionary of English.

• proper noun (Bot.) A genus of plants, one species of which (Ageratum Mexicanum) has lavender-blue flowers in dense clusters.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun uncountable Any of the genus Ageratum of tropical American herbs, annuals and perennials, from the sunflower family Asteraceae, tribe Eupatorieae; they are sometimes used in floral displays.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• 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
• noun any plant of the genus Ageratum having opposite leaves and small heads of blue or white flowers