The Collaborative International Dictionary of English v.0.48:

Cornflower \Corn"flow`er\ (k[^o]rn"flou`[~e]r), n. (Bot.)
   A conspicuous wild flower (Centaurea Cyanus), growing in
   grainfields.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Bachelor's button \Bach"e*lor's but"ton\, (Bot.)
   A plant with flowers shaped like buttons; especially, several
   species of Ranunculus, and the cornflower (Centaurea
   cyanus) and globe amaranth (Gomphrena).
   [1913 Webster]

   Note: Bachelor's buttons, a name given to several flowers
         "from their similitude to the jagged cloathe buttons,
         anciently worne in this kingdom," according to
         Johnson's --Gerarde, p. 472 (1633); but by other
         writers ascribed to "a habit of country fellows to
         carry them in their pockets to divine their success
         with their sweethearts." --Dr. Prior.
         [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Bluebottle \Blue"bot`tle\, blue-bottle \blue-bottle\, n.
   1. (Bot.) an annual Eurasian plant (Centaurea cyanus) which
      grows in grain fields; -- called also bachelor's button.
      It receives its name from its blue bottle-shaped flowers.
      Varieties cultivated in North America have showy heads of
      blue or purple or pink or white flowers

   Syn: cornflower, bachelor's button.
        [1913 Webster + WordNet 1.5]

   2. (Zool.) A large and troublesome species of blowfly (Musca
      vomitoria). Its body is steel blue.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Bluets \Blu"ets\, n. [F. bluet, bleuet, dim. of bleu blue. See
   Blue, a.] (Bot.)
   A name given to several different species of plants having
   blue flowers, as the Houstonia c[oe]rulea, the Centaurea
   cyanus or bluebottle, and the Vaccinium angustifolium.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Centaurea \Cen`tau*re"a\, prop. n. [NL. See Centaury.] (Bot.)
   A large genus of composite plants, related to the thistles
   and including the cornflower or bluebottle (Centaurea
   Cyanus) and the star thistle (Centaurea Calcitrapa).
   [1913 Webster]

WordNet (r) 3.0 (2006):

Centaurea cyanus
    n 1: an annual Eurasian plant cultivated in North America having
         showy heads of blue or purple or pink or white flowers
         [syn: cornflower, bachelor's button, bluebottle,
         Centaurea cyanus]