This relates to the frequency of failures which interrupt the operation of an instrument. It can be quantified, e.g. by the infrequency of breakdowns or by the availability of the instrument for use when required.
Source:
PAC, 1989, 61, 1657. 'Nomenclature for automated and mechanised analysis (Recommendations 1989)' on page 1663 (https://doi.org/10.1351/pac198961091657)