From 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]
From WordNet (r) 3.0 (2006) :
subgenus
n 1: (biology) taxonomic group between a genus and a species