Monophonic - Free meter - Modal - Latin - Neume - Gregorian chant

It is monophonic. This means there is only one voice. There is no harmony. The voices of the singers sing the same words at the same time with the same melody. The many voices are united into one.

In music, monophony is the simplest of musical textures, consisting of a melody (or "tune"), typically sung by a single singer or played by a single instrument player (e.g., a flute player) without accompanying harmony or chords. Many folk songs and traditional songs are monophonic. A melody is also considered to be monophonic if a group of singers (e.g., a choir) sings the same melody together at the unison (the exact same pitch) or with the same melody notes duplicated at the octave (such as when men and women sing together). If an entire melody is played by two or more instruments or sung by a choir with a fixed interval, such as a perfect fifth, it is also said to be monophony (or "monophonic"). The Musical texture of a song or musical piece is determined by assessing whether varying components are used, such as an accompaniment part or polyphonic melody lines (two or more independent lines).

It has free meter (rhythm). Nearly all other forms of music have a metrical rhythm. This means that there is a regular repeating rhythm. Think of dance music; it has to have a repeating, predictable beat. This is not the case with the vast bulk of the Gregorian repertory (the exceptions being office hymns and sequences). Rather, Gregorian chant has what is called free rhythm. The reason for this is that the music is wedded to the liturgical text, which is usually prose speech. Think of speech. Does it have a rhythm? Of course, but it is different from the regular reoccurring rhythm of, say, a clock ticking.

Modal music uses diatonic scales that are not necessarily major or minor and does not use functional harmony as we understand it within tonality. The term modal is most often associated with the eight church modes. The tonal center of these modes is called its "final."

The text is in Latin, language of the Roman Empire spread over Europe (the romances languages did not exist). These texts were taken of the Psalms and of other Ancient Testament books; some of them were taken from the Gospels and others were of own, generally anonymous inspiration. Nevertheless some liturgical pieces exist in Greek language: Kyrie eleison, Agios o Theos (Liturgy of Good Friday)

A neume (/ˈnjuːm/; sometimes spelled neum  is the basic element of Western and Eastern systems of musical notation prior to the invention of five-line staff notation. The word entered the English language in the Middle English forms "newme", "nevme", "neme" in the 15th century, from the Middle French "neume", in turn from either medieval Latin "pneuma" or "neuma," the former either from ancient Greek πνεῦμα pneuma ("breath") or νεῦμα neuma ("sign"), or else directly from Greek as a corruption or an adaptation of the former.