Although scientists continue to argue, we know what the planet is. They revolve around the Sun, are spherical, many have atmosphere... they are not decisive concepts, but at least they can be useful to explain what asteroids are.
Asteroids look curious. In the nearby photos we have obtained to date we see a surface of craters. They are of different sizes but below a limit. Characteristic of asteroids. If the mass were very large, it would round off gravity and gradually become planets. In addition, perhaps this gravity would not allow the gases of atmospheric formation to escape. In fact, the best known was the first to be discovered: Ceres. It has 1,000 kilometers from end to end and is a large asteroid. Asteroids 433 and 253 found are Eros and Matilde, respectively.