The Collaborative International Dictionary of English v.0.48:

Aleatory \A"le*a*to*ry\, a. [L. aleatorius, fr. alea chance,
   die.] (Law)
   Depending on some uncertain contingency; as, an aleatory
   contract. --Bouvier.
   [1913 Webster]

WordNet (r) 3.0 (2006):

aleatory
    adj 1: dependent on chance; "the aleatory element in life"

Moby Thesaurus II by Grady Ward, 1.0:

116 Moby Thesaurus words for "aleatory":
   Nachtmusik, absolute music, accidental, adaptation, adventitious,
   adventurous, air varie, aleatoric, aleatory music, amorphous,
   arrangement, blobby, blurred, blurry, broad, casual, causeless,
   chamber music, chamber orchestra, chance, chancy, chaotic,
   circumstantial, composition, conditional, confused, contingent,
   descant, destinal, dicey, disordered, electronic music, etude,
   exercise, fatal, fatidic, fluky, foggy, fortuitous, full of risk,
   fuzzy, general, harmonization, hazardous, hazy, hit-or-miss, iffy,
   ill-defined, imprecise, inaccurate, inchoate, incidental,
   incidental music, incoherent, indecisive, indefinable, indefinite,
   indeterminable, indeterminate, indistinct, inessential, inexact,
   instrumental music, invention, lax, loose, nocturne, nonessential,
   nonspecific, obscure, occasional, opus, orchestration, orderless,
   piece, production, program music, provisional, random, ricercar,
   riskful, risky, score, shadowed forth, shadowy, shapeless, sonata,
   sonatina, speculative, stochastic, string orchestra,
   string quartet, study, sweeping, theme and variations, trio,
   uncaused, unclear, undefined, undestined, undetermined,
   unessential, unexpected, unforeseeable, unforeseen, unlooked-for,
   unplain, unpredictable, unspecified, vague, variation, veiled,
   venturesome, venturous, wildcat, work