adjective

1. of or forming a foundation or basis; basic; essential the fundamental rules of art
2. relating to what is basic; radical a fundamental alteration
3. on which others are based; primary; original a fundamental type
4. most important; chief his fundamental needs
5. Music
   1. designating or of the lowest, or root, tone of a chord
   2. designating the prime or main tone of a harmonic series
6. Physics designating or of a fundamental

Etymology: LME < ML fundamentalis < L fundamentum: see fundament

noun

1. a principle, theory, law, etc. serving as a basis; essential part
2. Music
   1. the lowest, or root, tone of a chord
   2. the prime or main tone of a harmonic series
3. Physics the lowest frequency at which a system, as an air column or stretched string, will freely vibrate