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.
polysaccharide found in fruit and vegetables, crucial in forming jellies and jams, 1838, from French pectine, coined early 1830s by French chemist Henri Braconnot (1781-1855) from acide pectique "pectic acid," a constituent of fruit jellies, from Latinized form of Greek pektikos "curdling, congealing," from pektos "curdled, congealed," from pegnynai "to make stiff or solid," from PIE root *pag- "to fasten." Related: Pectic.
late 14c., Arrian, "adhering to the doctrines of Arius," from Late Latin Arianus, "pertaining to the doctrines of Arius," priest in Alexandria early 4c., who posed the question of Christ's nature in terms which appeared to debase the Savior's relation to God (denial of consubstantiation). Besides taking an abstract view of Christ's nature, he reaffirmed man's capacity for perfection. The doctrines were condemned at Nice, 325, but the dissension was widespread and split the Church for about a century during the crucial time of barbarian conversions. The name is Greek, literally "warlike, of Ares."