pivotal

pivotal    [Piv-uh-tl]

adjective.  of crucial importance, determining the future direction or turning point of something

"Thomas Jefferson played a pivotal role in the establishment of American equality."
"Chinese and American officials are meeting now at the State Department for a two-day talk.  It is clear that the relationship between the world's two most powerful countries has reached a pivotal moment."
"Antoine Vermette has scored four pivotal goals for the Chicago Blackhawks in the 2015 Stanley Cup Playoffs, with three of them being game-winners."