From The Collaborative International Dictionary of English v.0.48 [gcide]:
Crucial \Cru"cial\ (kr?"shal), a. [F. crucial, fr. L. crux,
crucis, cross, torture. See Cross.]
1. Having the form of a cross; appertaining to a cross;
cruciform; intersecting; as, crucial ligaments; a crucial
incision.
[1913 Webster]
2. Severe; trying or searching, as if bringing to the cross;
decisive; as, a crucial test.
[1913 Webster]
From WordNet (r) 3.0 (2006) [wn]:
crucial
adj 1: of extreme importance; vital to the resolution of a
crisis; "a crucial moment in his career"; "a crucial
election"; "a crucial issue for women" [syn: crucial,
important] [ant: noncrucial]
2: having crucial relevance; "crucial to the case"; "relevant
testimony"
3: of the greatest importance; "the all-important subject of
disarmament"; "crucial information"; "in chess cool nerves
are of the essence" [syn: {all-important(a)}, {all
important(p)}, crucial, essential, {of the essence(p)}]