(~s)
1.
If it is ~ that something is done, that thing is extremely important and must be done. (FORMAL)
It was ~ that he act as naturally as possible...
= vital
ADJ: usu v-link ADJ
2.
An ~ is something that is extremely important and must be done. (FORMAL)
The most important political ~ is to limit the number of US casualties.
N-COUNT: usu with supp
3.
In grammar, a clause that is in the ~, or in the ~ mood, contains the base form of a verb and usually has no subject. Examples are ‘Go away’ and ‘Please be careful’. Clauses of this kind are typically used to tell someone to do something.
N-SING: the N
4.
An ~ is a verb in the base form that is used, usually without a subject, in an ~ clause.
N-COUNT