The Collaborative International Dictionary of English v.0.48:

Subkingdom \Sub*king"dom\, n.
   One of the several primary divisions of either the animal, or
   vegetable kingdom, as, in Zoology, the Vertebrata, Tunicata,
   Mollusca, Articulata, Molluscoidea, Echinodermata,
   Coelentera, and the Protozoa; in botany, the Phanerogamia,
   and the Cryptogamia.
   [1913 Webster]

WordNet (r) 3.0 (2006):

subkingdom
    n 1: (biology) a taxonomic group comprising a major division of
         a kingdom