M42 Definition
abbreviation
(astronomy) The Orion Nebula; a diffuse nebula located at a distance of approximately 1600 light years from Earth, in the direction of the constellation Orion.
Wiktionary
Origin of M42
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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