**Pi** and **e** are examples of mathematical constants. A mathematical constant has the same value no matter which mathematical expression it appears in. Mathematical constants are special numbers–they pop up in many different mathematical expressions.

Take, for example, the constant **e** (Euler’s number), which equals 2.71828….Like pi, it goes on indefinitely. Euler’s number pops up in mathematical expressions in many different fields– compound interest, probability, the behavior of quantum particles, and so on.

Mathematical constants are bare numbers and imply no units of measurement. This is in contrast with physical constants such as **c**, the speed of light in a vacuum. Physical constants describe the size of a physical phenomenon and must have a unit of measurement attached. In the case of the speed of light, the unit of measurement is miles per second: **c** = 186,282 miles per second.