These words refer to the growing lack or need of something vital or to the turning point in an emergency or crisis. Crucial is the most general of these words in that it can refer to either of these situations equally well: a crucial battle that would determine the outcome of the whole war. By contrast, acute usually refers to a lack or need that has intensified to crisis proportion: an acute water shortage; an acute lack of lower-income housing. Less frequently, the word refers to other emergencies: the acute problem of air pollution in cities. In medical terminology, acute means grave or severe: an acute attack of bronchitis.

Critical is very similar to crucial in the way it refers either to an extreme lack or to a turning point: a critical absence of safety features in the new models; a critical showdown vote on the proposed legislation. The word differs from crucial in often suggesting a more exact measurement of the lack, especially when even a slight decline could have far-reaching consequences: Another minute’s lack of oxygen could be critical . The greater seriousness of critical as compared to crucial is also present when the reference is to a turning point. Where crucial may emphasize the absolute necessity that something happen in order to achieve a satisfactory result, critical more often suggests a balance between positive and negative outcomes that is beyond our power to influence: a decision that was to prove crucial to the great victory won that day; helpless to combat the critical weakening of the body’s defences. Critical can also be used to imply that something is scarce but vital: a list of critical materials.

Pressing and urgent are milder than the rest of these words. Pressing is especially so, since it can refer to a serious lack that is chronic rather than acute : a pressing need for changes that are not likely to be accomplished soon. Urgent suggests a nearer approach to crisis, but the lack it points to might be one among many, without necessarily being the decisive one: faced with so many urgent needs that they had to decide which of them were the critical ones and which could wait a little longer. Both pressing and urgent , moreover, can refer to an emergency without necessarily referring to its turning point: a pressing matter; an urgent conference between heads of state. Both words can refer to appeals rather than to the lack itself: a pressing request for funds; an urgent appeal for help. In all the various possibilities, however, urgent maintains its greater intensity when compared to pressing .