A nebula is a large cloud of interstellar gas or dust. In nontechnical use, the word nebula also refers to a galaxy other than the Milky Way.

// We were eventually able to see the nebula through the telescope.

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"Like clouds, the shapes of our galaxy’s glittery nebulae are sometimes in the eye of the beholder. They can look like all sorts of animals: tarantulas, crabs, a running chicken, and now, a cosmic koi swimming through space." — Laura Baisas, PopSci.com, 13 June 2024

The history of nebula belongs not to the mists of time but to the mists of Latin: in that language nebula means "mist" or "cloud." In its earliest English uses in the 1600s, nebula was chiefly a medical term that could refer either to a cloudy formation in urine or to a cloudy speck or film on the eye. Nebula was first applied to great interstellar clouds of gas and dust in the early 1700s. The adjective nebulous comes from the same Latin root as nebula, and it is considerably older, being first used as a synonym of cloudy or foggy as early as the 1300s. Like nebula, this adjective was not used in an astronomical sense until centuries later.