Pivotal (adj): Of the greatest possible importance. (केंद्रीय, आधारभूत)
Synonyms: Requisite, Indispensable, Elementary, Fundamental
Antonyms: Insignificant, Minor, Trivial, Unimportant
Example: The report was missing a pivotal piece of information.

Uses:

To understand why, a quick recap of pivotal moments of the past is unavoidable. The BNP won the first democratic polls in 1991, but refused to step down when their tenure ended in 1996.