M31 Definition
abbreviation
(astronomy) The Andromeda Galaxy; a giant spiral galaxy in the Local Group, together with the Milky Way galaxy. It is at a distance of approximately 2.5 million light years from Earth, in the direction of the constellation Andromeda.
Wiktionary
Origin of M31
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From Charles Messier, who first published Nebulae and Star Clusters in 1774 as a listing of visible objects in the night sky that were not stars. Objects in Messier's catalog are numbered, and the letter M (for Messier) is prepended to these numbers.
From Wiktionary