Mathematical constants are symbols that represent useful numbers.
τ (tau) is a geometric constant approximately equal to 6.283. The number is naturally occuring as any circle's circumference divided by its radius.
The greek letter π (pi) is a naturally occuring number that is defined by any circle's circumference divided by its diameter.
Euler's number is a naturally occuring number related to exponential growth and exponential decay.
The golden ratio is a number represented by the greek letter ϕ (phi). The value of ϕ is approximately 1.618 and is a naturally occuring number in nature. The golden ratio is often associated with the golden rectangle whose sides form a ratio equal to ϕ.
The letter "i" is used in mathematics to represent the square root of negative one. The letter is also used to denote the complex part of a complex number.