The Collaborative International Dictionary of English v.0.48:

Soybean \Soy"bean\ (soi"b[=e]n`), n.
   1. (Bot.) An Asiatic leguminous herb (Glycine max, formerly
      Glycine Soja) the seeds of which (also called soy
      beans) are used in preparing the sauce called soy.
      Called also soya bean and soya.
      [1913 Webster]

   2. the seeds of the Glycine max, which produce soybean
      oil; -- called also soya bean.
      [PJC]

WordNet (r) 3.0 (2006):

soybean
    n 1: a source of oil; used for forage and soil improvement and
         as food [syn: soy, soybean, soya bean]
    2: erect bushy hairy annual herb having trifoliate leaves and
       purple to pink flowers; extensively cultivated for food and
       forage and soil improvement but especially for its nutritious
       oil-rich seeds; native to Asia [syn: soy, soya,
       soybean, soya bean, soybean plant, soja, soja bean,
       Glycine max]
    3: the most highly proteinaceous vegetable known; the fruit of
       the soybean plant is used in a variety of foods and as fodder
       (especially as a replacement for animal protein) [syn: soy,
       soybean, soya, soya bean]