What is a galaxy? A great star system held together by gravity, with stars, gas and dust, black holes and dark matter. Astronomers used to think our Galaxy was the whole universe. But now that we know of billions of galaxies, they're considered to be the building blocks of the universe.
Not long after I'd written the galaxy article, there was an announcement about the biggest known spiral galaxy. It had always been considered a biggy, but getting an ultraviolet image showed an extended structure that we can't see in ordinary light. It's about five times as big as the Milky Way. Wow!