The Collaborative International Dictionary of English v.0.48:

Momentous \Mo*men"tous\, a. [Cf. L. momentosus rapid,
   momentary.]
   Of moment or consequence; very important; weighty; as, a
   momentous decision; momentous affairs. -- Mo*men"tous*ly,
   adv. -- Mo*men"tous*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

momentous
    adj 1: of very great significance; "deciding to drop the atom
           bomb was a very big decision"; "a momentous event"

Moby Thesaurus II by Grady Ward, 1.0:

93 Moby Thesaurus words for "momentous":
   absolute, ascendant, authoritarian, authoritative, authorized,
   autocratic, big, big-league, big-name, big-time, bigwig, bigwigged,
   bustling, charged, charismatic, charming, clothed with authority,
   commanding, competent, consequential, considerable, controlling,
   critical, crucial, decisive, dominant, double-barreled,
   duly constituted, earthshaking, effective, effectual, efficacious,
   eminent, empowered, enchanting, epochal, estimable, eventful,
   ex officio, fraught, full of incident, governing, grand, grave,
   great, heavyweight, hegemonic, hegemonistic, high-powered,
   imperative, important, influential, laden, leading, magnetic,
   major, material, meaningful, mighty, monocratic, name, of concern,
   official, personable, persuasive, phenomenal, pivotal, portentous,
   potent, powerful, preeminent, prestigious, prominent, puissant,
   ranking, reputable, ruling, self-important, senior, serious,
   significant, stirring, strong, suasive, substantial, superior,
   supreme, telling, totalitarian, vital, weighty, winning,
   world-shaking