As I get older, my chin folds. What are the distinct characteristics of a double chin?

I've noticed a thick layer of fat under my chin that wasn't there before. When I look in the mirror, it undeniably looks like a double chin, but I'm confused whether this is just sagging skin due to aging or if it is the "double chin" that others talk about. Since the members of this mom's cafe are all so knowledgeable about beauty, I'm asking here. Are there specific characteristics or criteria for a double chin that are usually defined by doctors? I'm curious if it has to fold when I lower my head, or if the area under the chin needs to bulge out even when I'm not looking. I need to know exactly what my condition is before I can start managing it, so if anyone knows a self-diagnosis method or how to distinguish a double chin, please leave a comment! ^^