A tributary is a smaller river or stream that flows into a bigger one. Note the direction — it always runs in, never out. The Han River is a tributary of the Yangtze; the water leaves the small river and joins the big one. Geography books use this word constantly, and so do news reports about floods and pollution, because what happens in a tributary ends up downstream in the main river. There is a second, older meaning, used as both noun and adjective: a tributary state or ruler was one that paid tribute — money, goods, or a formal gift — to a stronger power in exchange for being left alone. Imperial China's relationships with its neighbours are usually described in English as the tributary system, so Taiwanese students often meet this meaning in history before the river one. The two senses share one picture: something smaller giving what it has to something bigger. As an adjective it can also just mean 'flowing in' — a tributary stream.