Source : Webster's Revised Unabridged Dictionary (1913)

Subkingdom \Sub*king"dom\, n.
   One of the several primary divisions of either the animal, or
   vegetable kingdom, as, in zo["o]logy, the Vertebrata,
   Tunicata, Mollusca, Articulata, Molluscoidea, Echinodermata,
   C[oe]lentera, and the Protozoa; in botany, the Phanerogamia,
   and the Cryptogamia.

Source : WordNet®

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