Typically an astronomical (celestial) body refers to a single, cohesive structure that is bound together by gravity (and sometimes by electromagnetism). Examples include the asteroids, moons, planets and the stars.
Black holes are key to spiral arm hugs. The black hole at the center of the Triangulum Galaxy has a mass that is no more than times the mass of the sun. Its spiral arms are loosely wound at an angle of 43 degrees.