crucial
cru·cial
 /ˈkruːʃəl/  adj   [more ~; most ~]
  : extremely important
   Vitamins are crucial for maintaining good health.
   Eggs are a crucial [=essential] ingredient in this recipe.
   a crucial distinction/difference
   It's crucial that we arrive before 8 o'clock.
   Teachers are crucial to the success of the school.
   She played a crucial role in the meeting.
   Tonight's game is the crucial [=decisive] game of the series. [=whoever wins this game will win the series]
   a crucial decision
  cru·cial·ly  adv  
   crucially important