A billionth is one part of something divided into a billion equal pieces. If you split a meter into a billion tiny segments, each segment would be one billionth of a meter long (one nanometer). That's incredibly small: a human hair is about 100,000 nanometers wide, which means you could line up about 100,000 billionths of a meter across a single hair.

The word appears most often in science and technology, where measurements reach these microscopic scales. Computer chips contain parts measured in billionths of a meter. Scientists sometimes measure very short time spans in billionths of a second. Light travels about one foot in one billionth of a second.

You can also use billionth to describe position in a sequence: if you're the billionth person to visit a website, you're number 1,000,000,000 in line. When something happens for the billionth time, it's happened so often that you've completely lost count (though probably not literally a billion times).

The key to understanding billionth is grasping how enormous a billion is: if you counted one number per second without stopping, reaching a billion would take you over 31 years. So one billionth represents an almost unimaginably tiny fraction of the whole.