fundamental
fun-da-men-tal
adjective
Forming a necessary base or core; essential and primary to something's existence.
"Understanding basic maths is fundamental for science."
noun
A basic or primary rule, principle, or component that is essential.
"We must learn the fundamentals before advanced topics."
Thesaurus
Synonyms: rudimentary, underlying, basic, profound, significant, important, central, key, primal, of import, cardinal, first harmonic