fundamental

(noun): (usually in the plural) A leading or primary principle, rule, law, or article, which serves as the groundwork of a system; an essential part
(noun): The lowest frequency of a periodic waveform.
(noun): The lowest partial of a complex tone.
(adjective): Pertaining to the foundation or basis; serving for the foundation.
(adjective): Essential, as an element, principle, or law; important; original; elementary.