A formant is a frequency at which the vocal tract resonates. The F1 formant tends to shift with the height of the tongue body, with a low F1 signifying a high tongue body. The F2 formant tends to shift with the backness of the tongue body, with the difference between F2 and F1 indicating relative backness of the tongue body. A low F2-F1 signifies a tongue body that is back in the vocal tract.
Any of the constituents of a sound, especially a vowel sound, that impart to the sound its own special quality, tone colour, or timbre resonance of the vocal tract; a characteristic component of the quality of a sound, especially a speech sound; specifically any of several bands in which the frequencies subsidiary to the main frequency of a vowel sound are at their strongest and that differentiate the sound of one vowel from that of another.