I believe reliability means 'rate of success', i.e. it works reliably 0.8 or 80% of the time. So 1 is all the time, and 1-0.8=0.2 would be unreliability, which is related to the failure rate.

Look on the page for MIL-HDBK-217F

Reliability Prediction of Electronic Equipment- Notice F

http://www.weibull.com/knowledge/milhdbk.htm#200
One can download the pdf file.

Weibull.com is a good resource for statistics and statistical analysis.