A capillary is the smallest kind of blood vessel in the body — a tube so thin it is like a hair. Arteries bring blood from the heart, veins take it back, and capillaries are the net in between. Through their thin walls, oxygen and food slip out into the cells, and waste slips back in. You cannot see them the way you see a vein on your wrist. If a line stands out on your hand, that is a vein, not a capillary.

The same “thin as a hair” idea turns up in science class as capillary action: liquid climbing up a very thin tube by itself, even against gravity. That is how a paper towel drinks a spill, how ink finds a pen nib, and how water can rise in soil. Though actually, people say capillary action far more than they say a capillary tube in ordinary life. The “hair” picture is hiding in the Latin, not in the English spelling, so nobody guesses it without being told. This is not a fancy word for hair, and it is not a large vein.