crucial (adj.)

1706, "cross-shaped, having the form of an X," from French crucial, a medical term for ligaments of the interior of the knee-joint (which cross each other), from Latin crux (genitive crucis) "cross" (see crux).

The meaning "decisive, critical, finally disproving one of two alternative suppositions" (1830) is extended from a logical term, Instantias Crucis, adopted by Francis Bacon (1620); the notion is of cross fingerboard signposts at forking roads, thus a requirement to choose.

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Definitions of crucial from WordNet

crucial (adj.)
of the greatest importance;
crucial information
Synonyms: all-important / all important / essential / of the essence
crucial (adj.)
of extreme importance; vital to the resolution of a crisis;
a crucial election
a crucial moment in his career
a crucial issue for women
Synonyms: important
crucial (adj.)
having crucial relevance;
crucial to the case