Tonality is the arrangement of pitches and/or chords of a musical work in a hierarchy of perceived relations, stabilities, attractions and directionality. In this hierarchy, the single pitch or triadic chord with the greatest stability is called the tonic. The root of the tonic chord forms the name given to the key, so in the key of C major, the note C can be both the tonic of the scale and the root of the tonic chord (when it is C–E–G). The tonic can be a different note in the same scale, when the work is said to be in one of the modes of the scale.

Simple folk music songs often start and end with the tonic note. The most common use of the term "is to designate the arrangement of musical phenomena around a referential tonic in European music from about 1600 to about 1910". Contemporary classical music from 1910 to the 2000s may practice or avoid any sort of tonality—but harmony in almost all Western popular music remains tonal. Harmony in jazz includes many but not all tonal characteristics of the European common practice period, usually known as "classical music".

"All harmonic idioms in popular music are tonal, and none is without function." Tonality is an organized system of tones (e.g., the tones of a major or minor scale) in which one tone (the tonic) becomes the central point for the remaining tones. The other tones in a tonal piece are all defined in terms of their relationship to the tonic. In tonality, the tonic (tonal center) is the tone of complete relaxation and stability, the target toward which other tones lead. The cadence (a rest point) in which the dominant chord or dominant seventh chord resolves to the tonic chord plays an important role in establishing the tonality of a piece. "Tonal music is music that is unified and dimensional. Music is unified if it is exhaustively referable to a precompositional system generated by a single constructive principle derived from a basic scale-type; it is dimensional if it can nonetheless be distinguished from that precompositional ordering".

The term tonalité originated with Alexandre-Étienne Choron and was borrowed by François-Joseph Fétis in 1840. According to Carl Dahlhaus, however, the term tonalité was only coined by Castil-Blaze in 1821. Although Fétis used it as a general term for a system of musical organization and spoke of types de tonalités rather than a single system, today the term is most often used to refer to major–minor tonality, the system of musical organization of the common practice period. Major-minor tonality is also called harmonic tonality (in the title of Carl Dahlhaus, translating the German harmonische Tonalität), diatonic tonality, common practice tonality, functional tonality, or just tonality.