Exponential Distribution:
The exponential distribution is a onesided distribution completely specified by one parameter r > 0; the density of this distribution is

The mean of the exponential distribution is 1/r.
The exponential distribution is a model for the length of intervals between two consecutive random events in time, or between a given point and the next nearest point. In this case r is the rate of events  that is, the average number of events per a unit interval. (For details on what "random events" means here see Poisson process.
An important property of the exponential distribution is

Because of this property, the exponential distribution is also referred to as a ´memoryless distribution´. One of the applications of the exponential distribution is in queuing theory.
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