(Well, technically F0, but AFAIK you can't have HTML
elements in node titles.)
F0 (pronounced ef zero) is a term from phonetics which is
synonymous with fundamental frequency.
Why this abbreviation? Well, speech is commonly understood as a
sound with fundamental frequency and harmonics changing over time
due to the changing configuration of the vocal apparatus. The
supralaryngeal organs can amplify certain groups of frequencies, and
suppress others, giving rise to formants (peaks in the
frequency spectrum). The formants are numbered from lowest to highest,
thus: F1, F2, F3 ... Fn. By
overgeneralization, the fundamental frequency is looked upon as
"the zeroth formant", and called F0. The
fundamental frequency is not a formant, so it is technically
incorrect, but it is a convenient abbreviation, and saves four
syllables.