An avoid tone is a music theory term given to a scale degree which is considered especially dissonant relative to the harmony implied by the root chord. The root (basse fondamentale) of a chord is the note upon which that chord is perceived or labelled as built or centered, the root of a chord in root position or normal form. ...

For example, in C major, the root chord is C major triad (consisting of notes C, E, and G). Of the remaining notes in the scale, only F is considered "especially dissonant" to the root chord, so it is considered an avoid tone.

This term is usually discussed in terms of modal harmony. 'F' does not always sound dissonant in the key of C major, for instance if the underlying harmony is an F major chord. But for the purpose of discussion, avoid tones are considered relative to the root chord of the scale or mode. In the case of F major playing in the key of C major, you would discuss avoid tones relative to the scale with an F tonic, that is, F lydian mode.

Some scales (or modes) do not have any avoid tones. Examples are Lydian mode, Dorian mode.