fundamental

noun

1. (singular)A basic truth, elementary concept, principle, rule, or law. An individual fundamental will often serve as a building block used to form a complex idea.
2. (plural)A collection of essential component ideas that are often grouped together to serve as the foundational basis of a complex idea.

   “one of the fundamentals of linear algebra”

   “When any one offers me a compleat Catalogue of his Fundamentals, he does not unreaſonably demand me to quit mine for nothing[…]”

   “Personal leadership is a fundamental of successful government.”

3. The lowest frequency of a periodic waveform.
4. The lowest partial of a complex tone.

adj

1. Related to a foundation, base, or basis; serving as a foundation.
2. Essential; extremely important.

   “a fundamental truth; a fundamental axiom; a fundamental element; fundamental principle; fundamental law”

   “A need for belonging seems fundamental to humans.”

   “Seeing the British establishment struggle with the financial sector is like watching an alcoholic […]. Until 2008 there was denial over what finance had become. […] But the scandals kept coming, […]. A broad section of the political class now recognises the need for change but remains unable to see the necessity of a fundamental overhaul. Instead it offers fixes and patches.”