NOUN (2)

1. annual grass of Europe and North Africa; grains used as food and fodder (referred to primarily in the plural: `oats');

2. seed of the annual grass Avena sativa (spoken of primarily in the plural as `oats');

The Collaborative International Dictionary of English v.0.48:

Oat \Oat\ ([=o]t), n.; pl. Oats ([=o]ts). [OE. ote, ate, AS.
   [=a]ta, akin to Fries. oat. Of uncertain origin.]
   1. (Bot.) A well-known cereal grass (Avena sativa), and its
      edible grain, used as food and fodder; -- commonly used in
      the plural and in a collective sense.
      [1913 Webster]

   2. A musical pipe made of oat straw. [Obs.] --Milton.
      [1913 Webster]

   Animated oats or Animal oats (Bot.), A grass (Avena
      sterilis) much like oats, but with a long spirally
      twisted awn which coils and uncoils with changes of
      moisture, and thus gives the grains an apparently
      automatic motion.

   Oat fowl (Zool.), the snow bunting; -- so called from its
      feeding on oats. [Prov. Eng.]

   Oat grass (Bot.), the name of several grasses more or less
      resembling oats, as Danthonia spicata, Danthonia
      sericea, and Arrhenatherum avenaceum, all common in
      parts of the United States.

   To feel one's oats,
      (a) to be conceited or self-important. [Slang]
      (b) to feel lively and energetic.

   To sow one's wild oats, to indulge in youthful dissipation.
      --Thackeray.

   Wild oats (Bot.), a grass (Avena fatua) much resembling
      oats, and by some persons supposed to be the original of
      cultivated oats.
      [1913 Webster]

WordNet (r) 3.0 (2006):

oat
    n 1: annual grass of Europe and North Africa; grains used as
         food and fodder (referred to primarily in the plural:
         `oats')
    2: seed of the annual grass Avena sativa (spoken of primarily in
       the plural as `oats')