Answer:
1.
of vital importance; crucial.
"immediate action was imperative"
Similar:
vitally important
of vital importance
all-important
vital
crucial
critical
essential
of the essence
a matter of life and death
of great consequence
necessary
indispensable
exigent
pressing
urgent
required
compulsory
mandatory
obligatory
Opposite:
unimportant
optional
2.
giving an authoritative command; peremptory.
"the bell pealed again, a final imperative call"
