The Collaborative International Dictionary of English v.0.48:

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
   Compositae.
   [1913 Webster]

WordNet (r) 3.0 (2006):

suborder
    n 1: (biology) taxonomic group that is a subdivision of an order

Moby Thesaurus II by Grady Ward, 1.0:

55 Moby Thesaurus words for "suborder":
   biotype, blood, bracket, branch, caste, category, clan, class,
   division, estate, family, genotype, genus, grade, group, grouping,
   head, heading, kin, kingdom, label, level, order, phylum,
   pigeonhole, position, predicament, race, rank, rating, rubric,
   section, sept, series, set, species, station, status, strain,
   stratum, subclass, subdivision, subfamily, subgenus, subgroup,
   subkingdom, subspecies, subtribe, superclass, superfamily,
   superorder, superspecies, title, tribe, variety