An interstellar object is a non-stellar astronomical object not in closed orbit around a stellar object. Interstellar refers to the space between stars. Such objects are so far away, that they are not locked to a star (or stellar object) through gravitation.
Interstellar objects usually travel through space on a more or less straight line. When near a star, they travel too fast to remain nearby. They pass near a given star only once, not in a repeating orbit. Typical interstellar objects are similar to asteroids and comets.
Spacecraft (e.g. Voyager 1) that escape our star system are usually not called interstellar objects because they are not natural.