incipit

First few words of the opening line of a poem, song, or book, often used in lieu of a title

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". Its counterpart taken from the ending of the text is the explicit.