Constants

A fixed and well-defined mathematical object (often number), or the symbol denoting it. For example $\pi$ which equals to $3.1415...$ or just $42$.

• $\pi$ is the ratio of the circumference of a circle to its diameter with value approximately $3.14159$;
• $e$ is the number which derivative is equal to itself. It also has a lot of other different ways to describe it, for example via limits or infinite series. Its value approximately $2.71828$;
• $i$ is $\sqrt{-1}$ and used in calculations with complex numbers.