MTBF（平均故障间隔）是可靠度工程及制造工程学的名词，取自英文“Mean Time Between Failures”的缩写，意即是产品在操作使用或测试期间的平均连续无故障时间，需要注意的是，这里探讨的MTBF并非一个实测值，而是在产品设计阶段工程师依据理论所估算出的参考值。使用平均故障间隔时，一般假设故障的系统可以立刻修复。倘若故障的系统无法修复，一般改用MTTF（故障前平均时间）来说明。

平均故障间隔并非指系统一定出现物理性损坏，而是取决于该系统如何定义“故障”。例如对于需要高可靠性的复杂系统来说，“故障”就可能指的是系统出现预期以外的状况使得必须停止工作并维护。能通过良好的维护而加以避免，或者直接导致设备除役的“故障” 以及计划内必须使设备停止工作的维护并不在这个定义所考虑的范畴之内。

Mean time between failures (MTBF) is the predicted elapsed time between inherent failures of a system during operation. MTBF can be calculated as the arithmetic mean (average) time between failures of a system. The terms is used in both plant and equipment maintenance contexts. The MTBF is typically part of a model that assumes the failed system is immediately repaired (mean time to repair, or MTTR), as a part of a renewal process. This is in contrast to the mean time to failure (MTTF), which measures average time to failures with the modeling assumption that the failed system is not repaired (infinite repair time).