Mile per hour (mph) is a unit of speed commonly used in the United States and the United Kingdom. It measures the number of miles traveled in one hour. It is not an SI unit; the SI unit for speed is meters per second (m/s). One mile per hour is defined as exactly 0.44704 meters per second.
The mile itself has ancient roots, originating from the Roman "mille passus," or "a thousand paces." The mph unit gained prominence in the 19th century with the expansion of railroads. By the early 1900s, mph was widely adopted for road vehicles.
Mph is primarily used in the U.S. and the UK for speed limits on roads and signage. It is also employed in aviation and maritime contexts. In contrast, most other countries use kilometers per hour (km/h).