The United States customary system (also called American system or, more rarely, "English units") is the most commonly used system of measurement in the United States. It is similar but not identical to the British Imperial units. The U.S. is the only industrialized nation that does not mainly use the metric system in its commercial and standards activities,[1] although the International System of Units (SI, often referred to as "metric") is universally used in science, and increasingly in medicine, government, and many sectors of industry.