Definition: The Fahrenheit (symbol: °F) is a unit of temperature that was widely used prior to metrication. It is currently defined by two fixed points: the temperature at which water freezes, 32°F, and the boiling point of water, 212°F.

History & Origin: The Fahrenheit scale was developed based on a measurement proposed in 1724 by the German physicist Daniel Gabriel Fahrenheit. He initially based the scale on an equal ice-salt mixture, selecting the values of 30°F for the freezing point of water.

Current Use: Until the 1960s the Fahrenheit scale was the primary scale used in English-speaking countries. Today, the Fahrenheit scale is still used as the official temperature scale in a number of countries, including the United States, the Bahamas, and Belize.

Definition: The Celsius (symbol: °C) is an SI (International System of Units) derived unit of temperature. It is defined based on the SI unit of temperature, the kelvin. The Celsius and Kelvin scales are precisely related, with a one-degree change in Celsius being equal to a one degree-change in kelvin.

History & Origin: From 1743 until 1954, the Celsius scale was based on 0°C for the freezing point of water and 100 °C for the boiling point of water. In 1954, the unit was re-defined to instead be based on absolute zero (-273.15 °C) and the triple point of VSMOW.

Current Use: The Celsius scale replaced the Fahrenheit scale in most countries in the mid to late 20th century. Almost all countries around the world use this scale, except for those in which the metric system has not been adopted, such as the United States.