The constant c, the speed of light in a vacuum, is an example of a physical constant. It is a constant of nature, always 186,282 miles per second. Another example is the freezing point of pure water at sea level, 32 degrees Fahrenheit. A physical constant has the same value wherever it appears.

A physical constant is always accompanied by units of measurement such as miles per second, degrees, cycles per second, and so on. This contrasts with a mathematical constant, which is a bare number such as pi. Pi equals 3.14159…. No measurement units are implied.

Some physical constants are absolute, for example, the speed of light in a vacuum. It is always the same number. The freezing point of pure water at sea level is also an absolute constant, 32 degrees Fahrenheit.

Other physical constants vary with the situation. For more, see the section “Constants of nature that vary with the situation” in the definition of “absolute constant.”