In music theory, a scale degree is the name given to a particular note of a scale to specify its position relative to the tonic (the main note of the scale). The tonic is considered to be the first degree of the scale, from which each octave is assumed to begin.
Any musical scale may be thought to have degrees. However, the notion of scale degree is most commonly applied to scales in which a tonic is specified by definition, such as the 7-tone diatonic scales (e.g. the C-major scale C–D–E–F–G–A–B, in which C is the tonic). As for the 12-tone chromatic scale, the selection of a first degree is possible in theory, but arbitrary and not meaningful, because typically all the notes of a chromatic scale have the same importance.
The expression scale step is sometimes used as a synonym of scale degree, but it may also refer, perhaps more properly and less ambiguously, to the distance, or interval, between two successive scale degrees (see Steps and skips). Indeed, the terms whole step and half step are commonly used as interval names. The number of scale degrees and the distance between them together define a scale.
These names are derived from a scheme where the tonic note is the 'center'. Supertonic and subtonic are, respectively, one step above and one step below the tonic; mediant and submediant are each a third above and below the tonic, and dominant and subdominant are a fifth above and below the tonic.
^Benward & Saker (2003). Music: In Theory and Practice, Vol. I, p.32-3. Seventh Edition. ISBN 978-0-07-294262-0. "Scale degree names: Each degree of the seven-tone diatonic scale has a name that relates to its function. The major scale and all three forms of the minor scale share these terms."
^Jonas, Oswald (1982). Introduction to the Theory of Heinrich Schenker (1934: Das Wesen des musikalischen Kunstwerks: Eine Einführung in Die Lehre Heinrich Schenkers), p.22. Trans. John Rothgeb. ISBN 0-582-28227-6. Shown all uppercase.