fundamental

(usually in the plural) A leading or primary principle, rule, law, or article, which serves as the groundwork of a system; an essential part

"one of the fundamentals of linear algebra"

The lowest frequency of a periodic waveform.

The lowest partial of a complex tone.

Pertaining to the foundation or basis; serving for the foundation.

Essential, as an element, principle, or law; important; original; elementary.

"A need for belonging seems fundamental to humans."