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]

The Collaborative International Dictionary of English v.0.48:

Plantain \Plan"tain\, n. [Cf. F. plantain-arbre, plantanier, Sp.
   pl['a]ntano, pl['a]tano; prob. same word as plane tree.]
   1. (Bot.) A treelike perennial herb (Musa paradisiaca) of
      tropical regions, bearing immense leaves and large
      clusters of the fruits called plantains. See Musa.
      [1913 Webster]

   2. The fruit of this plant. It is long and somewhat
      cylindrical, slightly curved, and, when ripe, soft,
      fleshy, and covered with a thick but tender yellowish
      skin. The plantain is a staple article of food in most
      tropical countries, especially when cooked.
      [1913 Webster]

   Plantain cutter, or Plantain eater (Zool.), any one of
      several large African birds of the genus Musophaga, or
      family Musophagid[ae], especially Musophaga violacea.
      See Turaco. They are allied to the cuckoos.

   Plantain squirrel (Zool.), a Java squirrel (Sciurus
      plantani) which feeds upon plantains.

   Plantain tree (Bot.), the treelike herb Musa paradisiaca.
      See def. 1 (above).
      [1913 Webster]

WordNet (r) 3.0 (2006):

plantain
    n 1: any of numerous plants of the genus Plantago; mostly small
         roadside or dooryard weeds with elliptic leaves and small
         spikes of very small flowers; seeds of some used
         medicinally
    2: a banana tree bearing hanging clusters of edible angular
       greenish starchy fruits; tropics and subtropics [syn:
       plantain, plantain tree, Musa paradisiaca]
    3: starchy banana-like fruit; eaten (always cooked) as a staple
       vegetable throughout the tropics