What is the definition of 'diatonic' in relation to music?

The term 'diatonic' refers specifically to melody or harmony that is constructed using the notes that belong to a particular major or minor scale. This means that it is based on the seven diatonic tones that make up each scale. In other words, when music is described as diatonic, it includes only those pitches that fit into the established framework of a single major or minor scale without introducing additional notes from outside that scale (which would be chromatic tones). This relationship between diatonic music and the major and minor scales is fundamental, as it establishes the tonal center and the conventions of Western music theory that are widely accepted.

In contrast, the other choices describe concepts that do not align with the strict definition of diatonic music, such as melodies built from only a single note, which would not encompass the variety of notes inherent to the scales, or music that introduces chromatic tones that fall outside of the major or minor scale framework. Additionally, the mention of complex harmony with multiple keys pertains to more advanced harmonic structures and does not fit the simpler, scale-based definition of diatonic music.