I'm a fifteen year old who is currently studying circle geometry (if that is the appropriate term) and our teacher stated that concentric circles are similar. I thought about this, and it doesn't make sense to me. The reason is because of proportionality. For example, similar triangles are similar because they have the same angles and they have proportional sides. However, circles can not be compared for angles, so that's out (as they all have the same 360 degree angle at the center) and the only factor is their size, which is directly influenced by their radius. If the radius is the only variable involved in a triangle like this, how can a circle be NOT proportional to another circle? If a case of that existed, there would be meaning (at least from my current perspective) to the term "similar circle."
Help and critique on my logic is requested, and an explanation as to the term "similar circle."
You're right: any two circles are similar (and so there's not much point of talking about "similar circles")! In general, two shapes are "congruent" if you can turn one into the other by translations (moving around in the plane), rotations, and reflections. Two shapes are "similar" if you can rescale ("zoom in or out" on the picture) one of them to turn it into a shape that is congruent to the other. Given two circles, you can rescale one so that it has the same radius as the other, and then any two circles with the same radius are congruent since you can just translate the center of one to the center of the other.