The Collaborative International Dictionary of English v.0.48:

Pivotal \Piv"ot*al\, a.
   Of or pertaining to a pivot or turning point; belonging to,
   or constituting, a pivot; of the nature of a pivot; as, the
   pivotalopportunity of a career; the pivotal position in a
   battle.
   [1913 Webster]

WordNet (r) 3.0 (2006):

pivotal
    adj 1: being of crucial importance; "a pivotal event"; "Its
           pivotal location has also exposed it to periodic
           invasions"- Henry Kissinger; "the polar events of this
           study"; "a polar principal" [syn: pivotal, polar]

Moby Thesaurus II by Grady Ward, 1.0:

61 Moby Thesaurus words for "pivotal":
   acute, axial, behind the scenes, capital, cardinal, causal,
   causative, centermost, central, centralized, centric, centroidal,
   centrosymmetric, charged, clamorous, climacteric, compelling,
   concentrated, constitutive, critical, crucial, crying, decisive,
   determinative, effectual, emergent, essential, etiological,
   exigent, focal, formative, geocentric, high-pressure,
   high-priority, imperative, imperious, important, insistent,
   instant, institutive, kairotic, key, loaded, middle, middlemost,
   midmost, momentous, occasional, omphalic, originative, overriding,
   overruling, pregnant, pressing, principal, radical, ruling,
   significant, umbilical, urgent, vital