Metrication (or metrification) is the process of introducing the International System of Units, also known as SI units or the metric system, to replace a jurisdiction's traditional measuring units. Although U.S. customary units have been defined in terms of metric units since the 19th century, as of 2021 the United States is one of only three countries (the others being Myanmar and Liberia) that have not officially adopted the metric system as the primary means of weights and measures.