A number, approximately 2.718281828, that shows up absolutely everywhere in mathematics, including the limits of many infinite sums and continued fractions. It is the base of natural logarithms, which means that the area under the curve y=1/x between x=1 and x=e is exactly equal to one unit. e is important in probability and statistics, as well as number theory, analysis, and every other major branch of math. It is transcendental; it is related to pi by e^pi*i+1=0.