crucial

American Heritage® Dictionary of the English Language, Fourth Edition

1. adj. Extremely significant or important: a crucial problem.
2. adj. Vital to the resolution of a crisis; decisive: a crucial election. See Synonyms at decisive.
3. adj. Archaic Having the form of cross; cross-shaped.

Century Dictionary and Cyclopedia

1. Having the form of cross; transverse; intersecting; decussating: as, a crucial incision.
2. In anatomy, specifically applied to two stout decussating ligaments in the interior of the knee-joint, connecting the spine of the tibia with the intercondyloid fossa of the femur.
3. Decisive, as between two hypotheses; finally disproving one of two alternative suppositions. This meaning of the word is derived from Bacon's phrase instantia crucis, which be explains as a metaphor from a finger-post (crux). The supposed reference to a judicial “test of the cross,” as well as that to the testing of metals in a crucible, which different writers have thought they found in the expression, are unknown to as learned a lawyer and a chemist as Bacon and Boyle. These supposed derivations have, however, influenced some writers in their use of the word.
4. Of or pertaining to a crucible; like a heated crucible as a utensil of chemical analysis.
5. Pertaining to or like a cross as an instrument of torture for eliciting the truth; excessively strict and severe: said of a proceeding of inquiry.

Wiktionary

1. adj. Being essential or decisive for determining the outcome or future of something; extremely important.
2. adj. archaic Cruciform or cruciate; cross-shaped.
3. adj. slang Term of approval, particularly when applied to reggae music.

GNU Webster's 1913

1. adj. Having the form of cross; appertaining to a cross; cruciform; intersecting.
2. adj. Severe; trying or searching, as if bringing to the cross; decisive.

WordNet 3.0

1. adj. of the greatest importance
2. adj. of extreme importance; vital to the resolution of a crisis
3. adj. having crucial relevance