The lowest sound in a complex tone, and the one which is generally perceived as that complex tone's pitch.
The components of the complex tone which are higher in pitch than the fundamental are called "overtones". The more general term referring to a fundamental or any of its overtones is "partial".
With most acoustic musical instruments, the overtones have frequencies which are integer multiples of the fundamental, thus creating a harmonic series, which in turn helps to reinforce the perception of the fundamental pitch.
A term used to indicate the 1/1 of an otonality; that is, the low pitch that forms the 'root' of a chord composed of members from the harmonic series, all of which have a numerary nexus in the denominator of the fractions which describes the ratios.
The fundamental can always be found on a lattice diagram as a point in one of the corners of the structure. (See euler genus for examples.)