any of several plants (genus Ageratum) of the composite family with small, thick heads of usually bluish flowers: widely used as border plants in gardens
See ageratum in American Heritage Dictionary 4
(ăjˌə-rāˈtəm)
noun
Any of various New World plants of the genus Ageratum in the composite family, especially A. houstonianum, having showy, colorful flower heads.
Any of several other plants having flower clusters similar to the ageratum.