The Collaborative International Dictionary of English v.0.48:

Subgenus \Sub*ge"nus\, n.; pl. Subgenera. (Biol.)
   A subdivision of a genus, comprising one or more species
   which differ from other species of the genus in some
   important character or characters; as, the azaleas now
   constitute a subgenus of Rhododendron.
   [1913 Webster]

WordNet (r) 3.0 (2006):

subgenus
    n 1: (biology) taxonomic group between a genus and a species