Galaxy
A galaxy is an enormous system of stars, gas, dust, and dark matter held together by gravity, containing millions to trillions of stars.
Definition
A galaxy is a giant group of stars held together by gravity, along with gas, dust, and star remnants; our Sun lives in a galaxy called the Milky Way. Galaxies are classified by shape as spiral, elliptical, or irregular, and they range from dwarf galaxies with a few million stars to giants with trillions. The visible matter in a galaxy sits inside a much larger dark matter halo: rotation curves stay flat far from the center instead of falling off as $v \propto r^{-1/2}$, which is the primary evidence that most galactic mass is dark. Most large galaxies also host a supermassive black hole at the nucleus.
Example
The Andromeda Galaxy, about $2.5$ million light-years away, is the closest large galaxy to ours and is visible on a dark night as a faint smudge without a telescope. The Milky Way is a spiral about $100{,}000$ light-years across with a few hundred billion stars. Stars in the outer Milky Way orbit at roughly $220\ \text{km/s}$, about the same speed as stars far closer to the center; Keplerian motion around the visible mass alone would predict much slower outer speeds, so an unseen halo roughly ten times the visible mass must be present.
Key Insight
Every star you can see with your naked eye belongs to our own galaxy; other galaxies are so far away that their billions of stars blend into one tiny glow. Galaxies are mostly empty space: if the Sun were a grain of sand, the nearest star would be several kilometers away. Galaxies grow by merging, and the Milky Way and Andromeda are expected to merge in roughly $4$ billion years, though stars are so widely spaced that individual collisions will be rare.