adjective
basic, important
Synonyms: axiological, axiomatic, basal, bottom, bottom-line, cardinal, central, constitutional, constitutive, crucial, elemental, elementary, essential, first, foundational, grass-roots, indispensable, integral, intrinsic, key, major, meat-and-potatoes, necessary, organic, original, paramount, primary, prime, primitive, primordial, principal, radical, requisite, rudimentary, significant, structural, substratal, substrative, supporting, sustaining, theoretical, underived, underlying, vital
Antonyms: additional, advanced, auxiliary, extra, minor, secondary, subordinate, trivial, unimportant
noun
basic, essential part
Synonyms: ABCs, axiom, basis, bottom line, brass tacks, coal and ice, component, constituent, cornerstone, element, factor, foundation, guts, heart, law, nitty-gritty, principium, principle, rock bottom, rudiment, rule, sine qua non, theorem
Antonyms: addition, auxiliary, extra, subordinate, trivia