The definition of a time between failures (TBF) is a measure of the reliability of a system or component. It is the amount of time that passes between two consecutive failures of a system or component. TBF is an important metric for measuring the reliability of a system or component as it indicates how long a system or component can be expected to remain operational before it fails. TBF is usually measured in hours, days, or years, depending on the system or component being measured. A higher TBF value indicates a more reliable system or component, while a lower TBF value indicates a less reliable system or component. TBF is a key factor in determining the overall reliability of a system or component, and is often used to compare different systems or components in order to determine which one is more reliable.