The Collaborative International Dictionary of English v.0.48:

Subgroup \Sub"group`\, n. (Biol.)
   A subdivision of a group, as of animals. --Darwin.
   [1913 Webster]

WordNet (r) 3.0 (2006):

subgroup
    n 1: a distinct and often subordinate group within a group
    2: (mathematics) a subset (that is not empty) of a mathematical
       group

Moby Thesaurus II by Grady Ward, 1.0:

61 Moby Thesaurus words for "subgroup":
   adjunct, blood, bracket, branch, caste, category, clan, class,
   component, contingent, cross section, detachment, detail, division,
   dole, estate, fraction, grade, group, grouping, head, heading,
   installment, item, kin, label, level, order, parcel, part,
   particular, percentage, pigeonhole, portion, position, predicament,
   quadrant, quarter, quota, race, random sample, rank, rating,
   remainder, rubric, sample, sampling, section, sector, segment,
   sept, set, share, station, status, strain, stratum, subdivision,
   suborder, subspecies, title