What is a vessel legal definition

I : I assume you mean blood vessels. Blood vessels are the part of the circulatory system that transport blood throughout the body. There are three major types of blood vessels: 1)arteries, which carry blood away from the heart; 2) capillaries, which enable the actual exchange of water, oxygen and chemicals between the blood and the tissues; and 3) veins, which carry blood back toward the heart.

Legal definition !: Agree with Dr Whipple's very informative simple explanation of types of blood vessels, however you asked for a legal definition, we can't guess what you really mean, so kindly repost your question with more details, best wishes