What does m mean in m31?

In the context of M31, the "M" stands for "Messier," referring to the Messier catalog of astronomical objects compiled by the French astronomer Charles Messier in the 18th century. The number "31" indicates that it is the 31st object listed in this catalog. M31 specifically refers to the Andromeda Galaxy, which is the nearest spiral galaxy to the Milky Way and one of the brightest galaxies visible from Earth.