In music, a scale is any set of musical notes ordered by fundamental frequency or pitch. A musical scale represents a division of the octave space into a certain number of scale steps, a scale step being the recognizable distance (or interval) between two successive notes of the scale.
A specific scale is defined by its characteristic interval pattern and by a special note, known as its first degree (or tonic). The name of the scale specifies both its tonic and its interval pattern. For example, C-major indicates a major scale in which C is the tonic. The A major scale is written A–B–C♯–D–E–F♯–G♯, etc.
Therefore D-major is a noun, as scale is a noun. It's like saying, This is the person, Magus.
some scales are used in jazz and modern classical music, others in common in folk music, especially in oriental music; some have limited use in liturgy, others blues, jazz, etc.
Edited to add: Now I'm really confused.
a: having half steps between the third and fourth and the seventh and eighth degrees
b : based on a major scale
c : equivalent to the distance between the keynote and another tone (except the fourth and fifth) of a major scale
noun (b:) a major musical interval, scale, key, or mode
major noun (5.) Music. a major interval, chord, scale, etc.
- important, serious, or significant. antonyms: little, trivial, minor
- Music: (of a scale) having an interval of a semitone between the third and fourth degrees and the seventh and eighth degrees.
(of an interval) equivalent to that between the tonic and another note of a major scale, and greater by a semitone than the corresponding minor interval.
(of a key) based on a major scale, tending to produce a bright or joyful effect.
"Prelude in G Major"
1. an army officer of high rank, in particular...
2. Music: a major key, interval, or scale.
3. a student's principal subject or course of study.
4. a major world organization, company, or competition; the major leagues.
5. a person of full legal age.
6. Logic: a major term or premise.
7. Bridge: short for major suit.
1. specialize in (a particular subject) at a college or university.