A compound is something made by combining two or more separate parts into a single whole. In chemistry, a compound forms when different elements bond together: water is a compound of hydrogen and oxygen, and table salt is a compound of sodium and chlorine. The elements have chemically bonded into something new with its own properties, unlike mixtures that can be easily separated.

The word appears in many other contexts too. A compound word combines two words to make a new one: “butterfly” joins “butter” and “fly,” while “snowball” merges “snow” and “ball.” In math, compound interest means earning interest on your interest, so your money grows faster over time.

A compound can also be a fenced area containing buildings, like a family compound with several houses on one property, or a secure compound where diplomats live and work in a foreign country.

As a verb, to compound something means to make it worse by adding to it. If you forget your homework and then lie about it, you've compounded your mistake. The lie doesn't fix the original problem; it creates a bigger one.