Ambitus is a Latin term literally meaning "the going round", and in Medieval Latin means the "course" of a melodic line, most usually referring to the range of scale degrees attributed to a given mode, particularly in Gregorian chant. It may also refer to the range of a voice, instrument, or piece generally (Powers, Sherr, and Wiering 2001; Randel 2003). In Gregorian chant specifically, the ambitus is the range, or the distance between the highest and lowest note. Different chants vary widely in their ambitus. Even relatively florid chants like Alleluias may have a narrow ambitus. Earlier writers termed the modal ambitus "perfect" when it was a ninth or tenth (that is, an octave plus one or two notes, either at the top or bottom or both), but from the late fifteenth century onward "perfect ambitus" usually meant one octave, and the ambitus was called "imperfect" when it was less, and "pluperfect" when it was more than an octave (Powers, Sherr, and Wiering 2001).
All of the church modes are distinguished in part by their ambitus (Randel 2003). The plagal modes have the final in the middle of the ambitus, while the authentic modes generally go no more than one note below the final.