A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and maybe dark matter.
The word galaxy is derived from the Greek galaxias (γαλαξίας), literally «milky», a reference to the Milky Way. 
Galaxies range in size from dwarfs with just a few hundred million stars to giants with one hundred trillion stars, each orbiting its galaxy's center of mass.