From the Latin for "It begins" (incipere, infinitive*), an incipit is either:
A) the opening words or beginning of a text of a medieval manuscript or early printed book, or
B) the beginning of a medieval liturgical choral or other sacred vocal work.

The practice of Definition B stems from plainchant, when a solo voice would sing the opening line of a given section of text. For example, during the Gloria of the Catholic Mass, a solo voice, usually the cantor, would chant "Gloria in excelsis Deo," with the choir joining on "Et in terra pax". This would announce to the congregation that a new section of the Mass had started. A number of modern choral mass settings continue to use this practice, especially in the Gloria and Credo movements (the first lines "Gloria in excelsis Deo" and "Credo in unum Deum"), as a throwback to the origins of sacred choral music.

Many chant manuscripts are now categorized by their incipits, since most extant chant repertoire is anonymous. Medieval text manuscripts are also typically indexed by incipit.

*Latin lesson! Incipere, "to begin", also gives us roots for inception and incipient.