adjective

forming a necessary base or core; of central importance.

the protection of fundamental human rights

basic,foundational,rudimentary,elemental,elementary,underlying,basal,radical,root,primary,cardinal,initial,original,prime,first,primitive,primordial,principal,chief,capital,key,central,structural,organic,constitutional,inherent,intrinsic,ingrained,vital,essential,important,indispensable,necessary,crucial,pivotal,critical

noun

a central or primary rule or principle on which something is based.

two courses cover the fundamentals of microbiology

basics,essentials,rudiments,foundations,basic principles,first principles,preliminaries,crux,essence,core,nucleus,heart,base,bedrock,groundwork,crux of the matter,heart of the matter,sine qua non,nuts and bolts,nitty-gritty,brass tacks,ABC