When the word nebula entered English in the early 15th century, it had nothing to do with astronomy. Arriving as nebule meaning a cloud or mist from the Latin word nebula meaning mist, which in turn came from the Proto Indo-European root word *nebh-meaning cloud, vapor, fog, moist, sky. Ancient Greek had the related word nephele, nephos which also meant cloud. When the word nebula reappeared in English it had a medical meaning for cataracts or cloudy defects in the eye. The astronomical meaning of a cloud-like patch in the night sky was first recorded around 1730. It wasn’t until the early 20th century with the advent of modern and powerful telescopes that nebula were fully understood as massive clouds of gas and dust.
Image of the Bubble nebula, the Carina Nebula, the Lagoon Nebula and the 30 Doradus nebula, all courtesy NASA from Hubble Space Telescope Program.