crucial (adj.)

1706, "cross-shaped," from French crucial, a medical term for ligaments of the knee (which cross each other), from Latin crux (genitive crucis) "cross" (see cross (n.)). The meaning "decisive, critical" (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.