(1) The ability of a piece of equipment to perform its required function over a given period of time.

(2) The amount of time in a defined period during which the asset is able to provide the needed function; a measurement of total usable time during which an asset is not being serviced or otherwise in an inoperable state.

Source

(1) A Standardized Time Classification Framework for Mobile Equipment in Surface Mining: Operational definitions, Time Usage Model, and Key Performance Indicators 

(2) Guideline for the Implementation of Autonomous Systems in Mining  

Underground Mine Communications Infrastructure Guidelines Part III: General Guidelines