17. Euler's Number (e): 2.7182

Also known as the exponential growth constant, Euler’s number is the base for natural logarithms and is found in many areas of mathematics.

In finance, Euler's number is used to determine compound interest, which is extremely vital in understanding the time value of money which is the backbone of finance.

Also, Euler's number is crucial when describing any decaying relationship - think Carbon 14 dating.

16. Euler's Identity: e^(i*pi) = -1

Euler's identity is very important number. For example, we know from trigonometry that   e^ix = cosine(x) - i * sine(x)    because of the infinite series that describe sine, cosine, and e. When you put in pi for x, sine(pi) = 0 and cosine(pi) = -1.  It involves the most famous constants in math as well as 1, zero, and 1.

What this conceptually describes is walking along the edge of a unit circle in the complex plane, half way around. Until the real value is -1.