The exponential distribution function is defined as:

f(x) = l*e-lx

0 <= x < ∞

l > 0

where

l (lambda) |
is an exponential function parameter (an alternative parameterization is scale parameter b=1/l) |

e |
is the base of the natural logarithm, sometimes called Euler's e (2.71...) |

The graphic above shows the shape of the exponential distribution when lambda equals 1. For a complete listing of all distribution functions, see Distributions and Their Functions.