The Collaborative International Dictionary of English v.0.48:

Cumulative \Cu"mu*la*tive\ (k?"m?-l?-t?v), a. [Cf. F.
   cumulatif.]
   [1913 Webster]
   1. Composed of parts in a heap; forming a mass; aggregated.
      "As for knowledge which man receiveth by teaching, it is
      cumulative, not original." --Bacon
      [1913 Webster]

   2. Augmenting, gaining, or giving force, by successive
      additions; as, a cumulative argument, i. e., one whose
      force increases as the statement proceeds.
      [1913 Webster]

            The argument . . . is in very truth not logical and
            single, but moral and cumulative.     --Trench.
      [1913 Webster]

   3. (Law)
      (a) Tending to prove the same point to which other
          evidence has been offered; -- said of evidence.
      (b) Given by same testator to the same legatee; -- said of
          a legacy. --Bouvier. --Wharton.
          [1913 Webster]

   Cumulative action (Med.), that action of certain drugs, by
      virtue of which they produce, when administered in small
      doses repeated at considerable intervals, the same effect
      as if given in a single large dose.

   Cumulative poison, a poison the action of which is
      cumulative.

   Cumulative vote or Cumulative system of voting
      (Politics), that system which allows to each voter as many
      votes as there are persons to be voted for, and permits
      him to accumulate these votes upon one person, or to
      distribute them among the candidates as he pleases.
      [1913 Webster]

WordNet (r) 3.0 (2006):

cumulative
    adj 1: increasing by successive addition; "the benefits are
           cumulative"; "the eventual accumulative effect of these
           substances" [syn: accumulative, cumulative]

Moby Thesaurus II by Grady Ward, 1.0:

63 Moby Thesaurus words for "cumulative":
   absolute, accumulated, accumulative, additional, additive,
   additory, adducible, admissible, advancing, amassed, attestative,
   attestive, authentic, based on, certain, chain, circumstantial,
   conclusive, convincing, damning, decisive, determinative,
   documentary, documented, evidential, evidentiary, ex parte,
   eye-witness, factual, final, firsthand, founded on, grounded on,
   hearsay, heightening, implicit, incontrovertible, increasing,
   indicative, indisputable, intensifying, irrefutable, irresistible,
   magnifying, material, multiplying, nuncupative, overall,
   overwhelming, presumptive, probative, reliable, significant,
   snowballing, suggestive, summational, summative, sure, symptomatic,
   telling, total, valid, weighty



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

CUMULATIVE. Forming a heap; additional; as, cumulative evidence, or that
which goes to prove the same point which has been established by other
evidence. Cumulative legacy, or accumulative legacy, is a second bequest,
given by the same testator to the same legatee. 2 Rop. Log. 19,. See 1
Saund. 134, n. 4; Remedy.