e (Euler's Number)

The base of the natural logarithm, approximately 2.71828, and the unique number whose exponential function is its own derivative, which is why it appears throughout calculus, compound growth and decay, and probability. Jacob Bernoulli encountered the underlying limit implicitly in 1683 while studying compound interest taken to its continuous extreme, but it was Leonhard Euler who studied the constant systematically, proved it irrational, and gave it the letter e, first in unpublished work from 1727 to 1728 and then in his widely read 1748 Introductio in analysin infinitorum, which made the notation standard.