cru·cial / ˈkroōshəl/ • adj. decisive or critical, esp. in the success or failure of something: negotiations were at a crucial stage. ∎  of great importance: this game is crucial to our survival.DERIVATIVES: cru·ci·al·i·ty / ˌkroōshēˈalitē/ n.cru·cial·ly adv.ORIGIN: early 18th cent. (in the sense ‘cross-shaped’): from French, from Latin crux, cruc- ‘cross.’ The sense ‘decisive’ is from Francis Bacon's Latin phrase instantia crucis ‘crucial instance,’ which he explained as a metaphor from a crux or fingerpost marking a fork at a crossroad; Newton and Boyle took up the metaphor in experimentum crucis ‘crucial experiment.’

crucial cross-shaped XVIII; that decides between rival hypotheses, decisive XIX. — F., f. L. crux, cruc- CROSS 1; see -IAL.