The Collaborative International Dictionary of English v.0.48:

Epochal \Ep"o*chal\, a.
   Belonging to an epoch; of the nature of an epoch. "Epochal
   points." --Shedd.
   [1913 Webster]

WordNet (r) 3.0 (2006):

epochal
    adj 1: highly significant or important especially bringing about
           or marking the beginning of a new development or era;
           "epochal decisions made by Roosevelt and Churchill"; "an
           epoch-making discovery" [syn: epochal, epoch-making]

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "epochal":
   alternate, beating, circling, consequential, cyclic, even,
   every other, far-reaching, important, intermittent, isochronal,
   measured, metronomic, momentous, oscillatory, periodical, pulsing,
   reciprocal, recurrent, recurring, rhythmic, rotary, seasonal,
   serial, steady, undulant, undulatory, unmatched, unparalleled,
   wavelike, wheeling