I want to know exactly what varicose veins are.

As far as I know, varicose veins are when the blood vessels protrude and are easily visible to the naked eye, and I believe that is called varicose veins in the legs. I'm curious because I might be mistaken. I do exercise a lot, which causes my muscles to develop, and sometimes my blood vessels become very visible, but I wonder if that is indeed varicose veins. Are they harmful to the body? What kind of diagnostic methods are available? Please let me know.