The tonality of a composition.

System of notes or tones based on and named after the key note.

A group of pitches based on a particular tonic, and comprising a scale, regarded as forming the tonal basis of a piece or section of music.

A musical key is the relation of different chords to each other. The 'tonic' is the subjective sense of 'home', from which musical compositions deviate from, and arrive back to. Relations of different keys to each other give the impression of tension, development and resolution. A 'key signature' is an instruction at the beginning of written music, indicating what the 'home key' of the work is.