The Collaborative International Dictionary of English v.0.48:

Posterity \Pos*ter"i*ty\, n. [L. posteritas: cf. F.
   post['e]rit['e]. See Posterior.]
   1. The race that proceeds from a progenitor; offspring to the
      furthest generation; the aggregate number of persons who
      are descended from an ancestor of a generation;
      descendants; -- contrasted with ancestry; as, the
      posterity of Abraham.
      [1913 Webster]

            If [the crown] should not stand in thy posterity.
                                                  --Shak.
      [1913 Webster]

   2. Succeeding generations; future times. --Shak.
      [1913 Webster]

            Their names shall be transmitted to posterity.
                                                  --Shak.
      [1913 Webster]

            Their names shall be transmitted to posterity.
                                                  --Smalridge.
      [1913 Webster]

WordNet (r) 3.0 (2006):

posterity
    n 1: all of the offspring of a given progenitor; "we must secure
         the benefits of freedom for ourselves and our posterity"
         [syn: descendants, posterity]
    2: all future generations

Moby Thesaurus II by Grady Ward, 1.0:

66 Moby Thesaurus words for "posterity":
   aftermath, agnate, ancestry, backup man, blood, blood relation,
   blood relative, brood, children, clansman, cognate, collateral,
   collateral relative, conclusion, connections, consanguinean,
   consequence, descendant, descendants, distaff side,
   distant relation, dynasty, effect, enate, family, flesh,
   flesh and blood, folks, german, heir, heirs, inheritor, issue, kin,
   kindred, kinfolk, kinnery, kinsfolk, kinsman, kinsmen, kinswoman,
   kith and kin, line, lineage, near relation, next of kin, offspring,
   people, progeniture, progeny, relations, relatives, replacement,
   seed, sequel, sib, sibling, spear kin, spear side, spindle kin,
   spindle side, successor, successors, sword side, tribesman,
   uterine kin



Bouvier's Law Dictionary, Revised 6th Ed (1856):

POSTERITY, descents. All the descendants of a person in a direct line.



The Devil's Dictionary (1881-1906):

POSTERITY, n.  An appellate court which reverses the judgment of a
popular author's contemporaries, the appellant being his obscure
competitor.