As a noun, a compound is a thing made of two or more parts or, in chemistry, a substance of two or more elements joined together. As a verb, to compound means to make something worse by adding to it, or to mix things together.
From Latin 'componere' via Old French 'compondre', with the sense 'to put together'. The financial sense of 'compound interest' and the chemical sense both grow from this idea of combining.
Compound interest is called the 'eighth wonder of the world' because money that earns interest, which then earns more interest, grows explosively over time. In language, a 'compound word' like 'sunflower' is just two simple words fused into one powerful new idea.
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