The Roman form of early plain chant, as distinguished from the kindred Ambrosian, Gallican, and Mozarabic chants, which it gradually supplanted from the 8th to the 11th centuries.
Its name derives from Pope Gregory the Great (590-604) to whom tradition ascribes the final arrangement of the Roman chant.
Pius X, by his "Motu Proprio" of 22 November 1903, ordered the universal restoration of the authentic Gregorian is the sole chant of the Roman Church, describing it as the supreme type of sacred, music (which is one and the same as liturgical music) because it contains in the highest degree the qualities characteristic of sacred music: true art, and holiness.
It follows that Gregorian is altogether indispensable in the celebration of the solemn liturgy, since an integral part of that liturgy; so much so, in fact, that these liturgical functions cannot take place if the chant be lacking.
Such is the relation between the liturgy of the Church and the music it employs, as laid down by the papal code.
The now flourishing Pius X Institute of Liturgical Music was established in New York City for the purpose of working out systematically the reform in church music thus decreed.
New Catholic Dictionary