adjective

absolutely necessary; extremely important.

it is essential to keep up-to-date records

crucial,necessary,key,vital,indispensable,needed,required,called for,requisite,important,all-important,vitally important,of the utmost importance,of great consequence,of the essence,critical,life-and-death,imperative,mandatory,compulsory,obligatory,compelling,urgent,pressing,burning,acute,paramount,pre-eminent,high-priority,significant,consequential

noun

a thing that is absolutely necessary.

we only had the bare essentials in the way of equipment

necessity,necessary/essential item,prerequisite,requisite,requirement,need,condition,precondition,specification,stipulation,qualification,desideratum,sine qua non,must