in general, something that serves as a base or foundation for another.

in Music. A fundamental sound is that which forms the lowest note of the chord, and from whence are deduced the harmonical relations of the rest; or, which serves for a key to the tone. The fundamental bass is that which serves for a foundation to the harmony. A fundamental chord is that whose bass is fundamental, and in which the sounds are arranged in the same order as when they are generated, according to the experiment so often repeated by M. d’Alembert, in his Preliminary Discourse and Elements of Music. But as this order removes the parts to an extreme distance one from the other, they must be approximated by combinations or inversions; but if the bass remains the same, the chord does not for this reason cease to bear the name of fundamental. Such an example is this chord, ut mi sol, included in the interval of a fifth; whereas, in the order of its generation, ut sol mi, it includes a tenth, and even a seventeenth; since the fundamental ut is not the fifth of sol, but the octave of that fifth.