A constant is a number or symbol that always has the same value. There are four types of constants:

- A mathematical constant is a number like pi. Pi always equals 3.14159.

- An algebraic constant is a number which does not change in the context of a particular algebraic equation. In algebra, a constant is the opposite of a variable. For example,
**2**is an algebraic constant in the algebra equation**y = 2 + x**. This equation could also be written**y = a + x**if it were explained that**a**always equals**2****.**In which case,**a**would be an algebraic constant.

- A physical constant is a constant of nature like the speed of light in a vacuum. It’s an unchanging measurement: 186,282 miles per second and is represented by the symbol
**c**.

- A physical constant can also be one that changes with conditions, for example, the amount of kinetic energy for a specific number of molecules in a box at a specific temperature and pressure. The amount of kinetic energy is considered a constant but only for a certain number of molecules. For more, see the section “Constants of nature that vary with the situation” in the definition of “absolute constant.”