In music theory, a natural is an accidental which cancels previous accidentals and represents the unaltered pitch of a note.^[1]

A note is natural when it is neither flat (♭) nor sharp (♯) (nor double-flat  nor double-sharp ). Natural notes are the notes A, B, C, D, E, F, and G represented by the white keys on the keyboardof a piano or organ. On a modern concert harp, the middle position of the seven pedals that alter the tuning of the strings gives the natural pitch for each string.

The scale of C major is sometimes regarded as the central, natural or basic major scale because all of its notes are natural notes, whereas every other major scale has at least one sharp or flat in it.

The notes F♭, C♭, E♯, B♯, and most notes inflected by double-flats and double-sharps correspond in pitch with natural notes; however, they are not regarded as natural notes but rather as enharmonic equivalents of them and are just as much chromatically inflected notes as most sharped and flatted notes that are represented by black notes on a keyboard.

https://en.m.wikipedia.org/wiki/Natural_(music)

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Christoph Willibald (Ritter von) Gluck (German: [ˈkʁɪstɔf ˈvɪlɪbalt ˈɡlʊk]; 2 July 1714 – 15 November 1787) was a composer of Italian and French opera in the early classical period. Born in the Upper Palatinate (now part of Germany) and raised in Bohemia,[1] he gained prominence at the Habsburg court at Vienna, where he brought about the practical reform of opera’s dramaturgical practices that many intellectuals had been campaigning for over the years. With a series of radical new works in the 1760s, among them Orfeo ed Euridice and Alceste, he broke the stranglehold that Metastasian opera seria had enjoyed for much of the century.

Source: Christoph Willibald Gluck – Wikipedia