If a sound is a complex of many tones of various frequencies, amplitudes and phases, repeating together in a basic cycle of definite frequency, the fundamental is the lowest frequency of this complex and corresponds to the unique pitch heard in such a complex tone.
The fundamental does not necessarily have the greatest amplitude, however, and even if missing (such as in the case of a low voice speaking through the carbon microphone of a telephone) the brain will still identify that frequency as the fundamental, called the periodicity pitch or missing fundamental. This ability of the auditory system is called fundamental tracking. (Source - Barry Truax - Handbook for Acoustic Ecology CD-ROM Edition. Cambridge Street Publishing, 1999 - CSR-CDR 9901)