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]

The Collaborative International Dictionary of English v.0.48:

momentousness \mo*men"tous*ness\ n.
   utmost importance.
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

momentousness
    n 1: utmost importance