(1) The length of time it takes for drilling [[mud]] to travel from a location on the surface to the bottom end of the [[drill stem]] (assembly of drill pipe, etc.) during a drilling operation. It is a function of the inside diameter and length of drill pipe and the output of the mud pump.

(2) The length of time lost during an operation because of non-scheduled stoppages such as failure, delay, etc.

(3) The period during which an item of equipment cannot be operated because of ongoing repair or maintenance.