imperative

Collins Cobuild// 

  1. ADJ : usu v-link ADJ: 

      If it is imperative that something is done, that thing is extremely important and must be done.  [FORMAL]

• It was imperative that he act as naturally as possible.     
• = vital   

  2. N-COUNT : usu with supp: 

     An imperative is something that is extremely important and must be done. [FORMAL]

• The most important political imperative is to limit the number of US casualties.
  

Am Heritage//

  adj 1. Expressing a command or plea; peremptory: requests that grew more and more imperative.

  adj 4. Impossible to deter or evade; pressing: imperative needs. (See Synonyms at urgent.)

  noun 1(a). A command; an order. 1(b). An obligation; a duty: social imperatives.

  cf. adj 2. Having the power or authority to command or control.

  cf. noun 2. A rule, principle, or instinct that compels a certain behavior: 

• a people driven to aggression by territorial imperatives.