In Information theory, outage probability of a communication channel is the probability that a given information rate is not supported, because of variable channel capacity.
It is the probability that an outage will occur within a specified time period.
[1] For example, the channel capacity for slow-fading channel is C = log2(1 + h2 SNR), where h is the fading coefficient and SNR is a signal to noise ratio without fading.
There may be a chance that information rate may go below to required threshold level.
For slow fading channel, outage probability = P(C < r) = P(log2(1 + h2 SNR) < r), where r is the required threshold information rate.