'M31' originates from French, specifically from the surname 'Messier' (Charles Messier), where 'Messier' is the name of the astronomer who compiled the catalog; the 'M' stands for 'Messier' and '31' is the object's number in that catalog.

'M31' changed from earlier references such as the descriptive phrase 'Andromeda Nebula' in 18th-century records; it was cataloged as 'M 31' by Charles Messier in the Messier catalogue and later cross-identified with 'NGC 224' in the New General Catalogue as astronomical understanding advanced.

Initially it referred to a faint 'nebula' recorded as the 31st entry in Messier's list; over time, as extragalactic astronomy developed, the designation came to be specifically associated with the Andromeda Galaxy, recognized as a separate galaxy outside the Milky Way.