9
In common usage, nine is an integer number and one of the digits in the decimal system.
In probability, the nine is a logarithmic measure of probability of an event, defined as the negative of the base 10 logarithm of the probability of the event's complement. For example, an event that is 99 percent likely to occur has 0.01 (1 percent) unlikelihood, or -log10(0.01) = 2 nines of probability.
Availability of a system, which refers to the probability that a system is available at any given moment, is often measured in nines. Many computer and communications facilities strive toward five nines (99.999 percent) availability, which implies a total downtime of no longer than five minutes per year.
The effectiveness of spam filters has also been stated in nines.