Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

pivotal
     adj : 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: {polar}]