Source : Webster's Revised Unabridged Dictionary (1913)

Suborder \Sub*or"der\, n. (Nat. Hist.)
   A division of an order; a group of genera of a little lower
   rank than an order and of greater importance than a tribe or
   family; as, cichoraceous plants form a suborder of
   Composit[ae].

Source : WordNet®

suborder
     n : (biology) taxonomic group that is a subdivision of an order