Heavyweight originally described boxers in one of the largest weight classes in boxing, usually over about 200 pounds in modern professional boxing. When two heavyweight champions fight, they're often among the biggest, strongest boxers in the world. Famous heavyweight boxers like Muhammad Ali and Joe Louis became household names because heavyweight matches drew enormous crowds and attention.

The word has grown beyond boxing to mean anyone or anything with exceptional power, influence, or importance in their field. A heavyweight politician might be a senator whose opinions shape national policy. A heavyweight technology company like Apple or Microsoft influences how millions of people work and communicate. When someone calls a scientist a heavyweight in her field, they mean her research and ideas carry serious weight with other experts.

You can also use heavyweight to describe someone who's simply large, though this usage is less common and can sound unkind. The opposite of a heavyweight is a lightweight, which in boxing means a smaller weight class but in general usage suggests someone with less power, skill, or influence. When people describe someone as “a real heavyweight,” they're emphasizing that this person's abilities or position make them impossible to ignore.