The Collaborative International Dictionary of English v.0.48:

Plantain \Plan"tain\, n. [F., fr. L. plantago. Cf. Plant.]
   (Bot.)
   Any plant of the genus Plantago, but especially the
   Plantago major, a low herb with broad spreading radical
   leaves, and slender spikes of minute flowers. It is a native
   of Europe, but now found near the abode of civilized man in
   nearly all parts of the world.
   [1913 Webster]

   Indian plantain. (Bot.) See under Indian.

   Mud plantain, a homely North American aquatic plant
      (Heteranthera reniformis), having broad, reniform
      leaves.

   Rattlesnake plantain, an orchidaceous plant (Goodyera
      pubescens), with the leaves blotched and spotted with
      white.

   Ribwort plantain. See Ribwort.

   Robin's plantain, the Erigeron bellidifolium, a common
      daisylike plant of North America.

   Water plantain, a plant of the genus Alisma, having acrid
      leaves, and formerly regarded as a specific against
      hydrophobia. --Loudon.
      [1913 Webster]

WordNet (r) 3.0 (2006):

mud plantain
    n 1: grassy-leaved North American aquatic plant with yellow
         star-shaped blossoms [syn: water star grass, mud
         plantain, Heteranthera dubia]