Mean time between failures (MTBF) is the predicted elapsed time between inherent failures of a system during operation.
Due to the very low failure rate of integrated circuits, it is convenient to refer to failures in a population during a period of 10e9 device hours; one failure in 10e9 device hours is defined as one FIT. The reliability report provides the FIT data and MTBF=1/FIT:
http://www.latticesemi.com/corporate/newscenter/newsletters/newsfebruary2009/reliabilitymonitorreport.cfm