bachelor's button

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

• noun Any of several plants that have buttonlike flowers or flower heads.

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

• (Bot.) A plant with flowers shaped like buttons; especially, several species of Ranunculus, and the cornflower (Centaurea cyanus) and globe amaranth (Gomphrena).

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

• noun Any one of a number of plants with daisylike, button shaped flowers of the genus Centaurea, especially the species Centaurea cyanus.
• noun A button which may be attached to clothing without resorting to needle and thread.

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

• noun tropical American herb having rose to red or purple flowers that can be dried without losing color
• noun an annual Eurasian plant cultivated in North America having showy heads of blue or purple or pink or white flowers