e

   1. a symbol for the electric charge on one electron. Since the charges on other particles in atomic physics are whole-number multiples of this charge, the symbol e is often used as a unit of measure. 1 e is equal to approximately 1.602 176 487 x 10^-19 coulomb, or 160.217 648 7 zeptocoulombs (zC).

   2. a mathematical unit used as the base of "natural" logarithms and exponentials.

   The real number e is irrational, which means that its decimal expansion is infinite and non-repeating. To 25 significant digits e equals 2.718 281 828 459 045 235 360 287. Of the many properties of this number, the most important is that the rate of change in the function e^x is equal to the value of the function itself: an example of the behavior we call "exponential growth." As a result, the larger the value of this function is, the faster the function grows. The Swiss mathematician Leonard Euler (1707-1783) introduced the symbol e, probably because it is the first letter of the word "exponential." Other mathematicians continued to use the letter in his honor. It is sometimes called the Euler number.