A bioindicator is a living thing — a plant, an animal, an insect, sometimes something too small to see — that scientists watch in order to learn about a whole place. They do not try to count every fish in the river. They pick a few kinds that belong there when the place is well, and that change when the place is in trouble, and those few stand in for the rest. If certain water insects vanish, the river is often dirty. If certain lichens grow on the trees, the air is often fairly clean. A short dictionary line will say they are “typical” of the life in that area, and that is part of it: they are the usual company of a healthy place. Though actually the useful part is not that they are average, but that they react early. They get sick, leave, or grow well before the rest of the picture is obvious. You will meet the word in ecology and in news about dirty rivers or dirty air, not in ordinary chat. It names the living thing, or a small set of them — not a machine and not a chemical test.