"funds essential to the completion of the project"
"an indispensable worker"
indispensable not to be dispensed with; absolutely necessary; unavoidable
essential as in basic and fundamental
"the essential feature"
essential as in of the greatest importance
"the all-important subject of disarmament"
"in chess cool nerves are of the essence"
crucialof extreme importance; having crucial relevance; of the greatest importance
of the essence(p)
essential as in being or relating to or containing the essence of a plant etc
essential as in defining rights and duties as opposed to giving the rules by which rights and duties are established
substantive any word or group of words functioning as a noun ; of or relating to the real nature or essential elements of something; defining rights and duties as opposed to giving the rules by which rights and duties are established; being on topic and prompting thought