The Collaborative International Dictionary of English v.0.48:

Gillyflower \Gil"ly*flow`er\, n. [OE. gilofre, gilofer, clove,
   OF. girofre, girofle, F. girofle: cf. F. girofl['e]e
   gillyflower, fr. girofle, Gr. ? clove tree; ? nut + ? leaf,
   akin to E. foliage. Cf. Caryophyllus, July-flower.]
   [Written also gilliflower.] (Bot.)
   1. A name given by old writers to the clove pink (Dianthus
      Caryophyllus) but now to the common stock (Matthiola
      incana), a cruciferous plant with showy and fragrant
      blossoms, usually purplish, but often pink or white.
      [1913 Webster]

   2. A kind of apple, of a roundish conical shape, purplish red
      color, and having a large core.

   Clove gillyflower, the clove pink.

   Marsh gillyflower, the ragged robin (Lychnis
      Flos-cuculi).

   Queen's gillyflower, or Winter gillyflower, damewort.

   Sea gillyflower, the thrift (Armeria vulgaris).

   Wall gillyflower, the wallflower (Cheiranthus Cheiri).

   Water gillyflower, the water violet.
      [1913 Webster]

WordNet (r) 3.0 (2006):

water gillyflower
    n 1: a featherfoil of the eastern United States with submerged
         spongy inflated flower stalks and white flowers [syn:
         water gillyflower, American featherfoil, Hottonia
         inflata]