⁴⁴MTBF stands for Mean Time Between Failure, and represents the reliability of a large collection of components or systems. For any large batch of identical components or systems constantly subjected to ordinary stresses, MTBF is the theoretical length of time it will take for 63.2% of them to fail based on ordinary failure rates within the lifetime of those components or systems. Thus, MTBF may be thought of as the “time constant” (τ) for failure within a batch of identical components or systems.