The Messier Catalog (or list of Messier Objects or Catalogue des Nebuleuses et des Amas d'Etoiles, i.e., Catalogue of Nebulae and Star Clusters) is Charles Messier's 1771 catalog of fixed but apparently-non-stellar astronomical objects, of which most are now known to be galaxies. Messier was interested in finding Comets and produced this list of objects that look like comets but had been established to be something else. The objects are designated M with a number (examples: Andromeda (M31), Triangulum Galaxy (M33), M82, M87) and include galaxies, clouds, and clusters of stars. All that can be currently identified are also in the New General Catalogue (NGC). The Messier designations are still often used by astronomers.
Whirlpool Galaxy (M51a)