Douglas Harper's Etymology Dictionary
opening word of a Latin book or manuscript, Latin, literally "(here) begins," third person singular present indicative of incipere (see incipient).
n. The first few words of a text, especially its first line.
The incipit of a text is the first few words of the text, employed as an identifying label. In a musical composition, an incipit is an initial sequence of notes, having the same purpose. The word incipit comes from Latin and means "it begins".
Before the development of titles, texts were often referred to by their incipits, as with for example Agnus Dei. During the medieval period in Europe, incipits were often written in a different script or colour from the rest of the work of which they were a part, and "incipit pages" might be heavily decorated with illumination. Though the word incipit is Latin, the practice of the incipit predates classical antiquity by several millennia and can be found in various parts of the world. Although not always called by the name of incipit today, the practice of referring to texts by their initial words remains commonplace.