If something happens or is located in between stars, it's interstellar. If you dream of interstellar travel, you're imagining flying through space — and maybe meeting a few extraterrestrials along the way.
The area inside a galaxy is often referred to by astronomers as interstellar space. In fact, the adjective interstellar is frequently used in a general way as a synonym for "outer space" or "between solar systems." So if an astronaut is planning an interstellar trip, she may be traveling in space, but she's not literally making a journey from one star to another. The roots of interstellar are inter-, "between," and the Latin stella, or "star."