One billionth of a meter. For comparison, a dime is about 1.3 millimeters thick. If you take that dime and divide it to one-one thousandth of its thickness, you get a micron. A micron is about as small as you can see with the best optical microscope (a human blood cell is five to 10 microns in diameter). A nanometer is a thousandth of a micron, or 10^-9 meter.