The Collaborative International Dictionary of English v.0.48:

Campion \Cam"pi*on\, n. [Prob. fr. L. campus field.] (Bot.)
   A plant of the Pink family (Cucubalus bacciferus), bearing
   berries regarded as poisonous.
   [1913 Webster]

   Bladder campion, a plant of the Pink family (Cucubalus
      Behen or Silene inflata), having a much inflated calyx.
      See Behen.

   Rose campion, a garden plant (Lychnis coronaria) with
      handsome crimson flowers.
      [1913 Webster]

WordNet (r) 3.0 (2006):

bladder campion
    n 1: perennial of Arctic Europe having large white flowers with
         inflated calyx [syn: bladder campion, Silene uniflora,
         Silene vulgaris]
    2: bluish-green herb having sticky stems and clusters of large
       evening-opening white flowers with much-inflated calyx;
       sometimes placed in genus Lychnis [syn: white campion,
       evening lychnis, white cockle, bladder campion, Silene
       latifolia, Lychnis alba]