crucial,
important of extreme importance; vital to the resolution of a crisis; "a crucial moment in his career"; "a crucial election"; "a crucial issue for women"
of import,
important of great significance or value; "important people"; "the important questions of the day"
indispensable unavoidable; "the routine but indispensable ceremonies of state"
intrinsical,
intrinsic situated within or belonging solely to the organ or body part on which it acts; "intrinsic muscles"
necessary unavoidably determined by prior circumstances; "the necessary consequences of one's actions"
primary not derived from or reducible to something else; basic; "a primary instinct"
must highly recommended; "a book that is must reading"
no-frills characterized by the absence of inessential features; "he got a no-frills introduction to the job"
staple necessary or important, especially regarding food or commodities; "wheat is a staple crop"
substantive,
substantial defining rights and duties as opposed to giving the rules by which rights and duties are established; "substantive law"
virtual existing in essence or effect though not in actual fact; "a virtual dependence on charity"; "a virtual revolution"; "virtual reality"
essential -
absolutely necessary; vitally necessary; "essential tools and materials"; "funds essential to the completion of the project"; "an indispensable worker"
