Fundamental

The fundamental is the lowest pitch to which a series of overtones belong.

If a scale has an equave and is based to some degree on the harmonic series, then the tonic of that scale is likely to be equave-equivalent to the fundamental of that harmonic series.

Depending on the scale, tuning, or other structure being analysed, the fundamental may be interpreted as different things numerically. Most often, though, it is interpreted as 1/1, the perfect unison, or as a much lower equave-equivalent version of that, such as 1/2, 1/4, 1/8 etc. for octave equivalence, or 1/3, 1/9, 1/27 etc. for tritaves, 1/5, 1/25, etc. for pentaves, so on.