Essential; crucial; extremely important.
Having semantics that incorporates mutable variables.
Expressing a command; authoritatively or absolutely directive.
Of, or relating to the imperative mood.
A verb in imperative mood.
The grammatical mood expressing an order (see jussive). In English, the imperative form of a verb is the same as that of the bare infinitive.
An essential action, a must: something which is imperative.
Absolutely necessary or requisite; that one cannot do without.
Unbendable, that cannot be set aside or ignored.
A thing that is not dispensable; a necessity.