Definition: Miles per hour (symbol: mph) is a unit of speed expressing the number of miles traveled in one hour. It is part of the imperial and US customary systems and is used primarily in the United States and a few other countries.

History/origin: The mph unit comes from the traditional imperial system where distances were measured in miles. One mile is defined as exactly 1,609.344 meters. The mph unit has been used historically for transportation, legal speed limits, and aviation.

Current use: Miles per hour is used predominantly in the United States, the United Kingdom, and a few Caribbean nations. It is the standard unit for speed on roadways, in cars, and in aviation for indicating airspeed.

Definition: Kilometers per hour (symbol: km/h or kph) is a unit of speed expressing the number of kilometers traveled in one hour. It is derived from the metric system and is commonly used to measure vehicle speeds, especially in countries that use the metric system.

History/origin: The kilometer per hour originated from the base metric units of kilometers (distance) and hours (time). As the metric system became widely adopted in the 19th and 20th centuries, kph became a standard unit for speed in most of the world.

Current use: Kph is the primary speed unit used in most countries, including Europe, Asia, Africa, and South America. It is used on road signs, vehicle speedometers, and for reporting weather data such as wind speeds.