Fundamental

Part of speech: noun (n.)

A leading or primary principle, rule, law, or article, which serves as the groundwork of a system; essential part, as, the fundamentals of the Christian faith.

Part of speech: adjective (a.)

Pertaining to the foundation or basis; serving for the foundation. Hence: Essential, as an element, principle, or law; important; original; elementary; as, a fundamental truth; a fundamental axiom. The fundamental reasons of this war. Shak. Some fundamental antithesis in nature. Whewell. Fundamental bass (Mus.), the root note of a chord; a bass formed of the roots or fundamental tones of the chords. -- Fundamental chord (Mus.), a chord, the lowest tone of which is its root. -- Fundamental colors, red, green, and violet-blue. See Primary colors, under Color.