Operational tempo, known in milspeak as optempo, is a phrase that has been surfacing more and more frequently in the U.S. defense establishment over the latter half of the 1990s and on. It is misused as often as it is used correctly, and by serving officers as well. The Department of Defense takes a stab at a definition on its DefenseLINK website:

"A tempo is a musical term meaning the speed at which a piece is played. The word's definition over time has expanded to mean the pace of an activity, as in the 'tempo of everyday life is getting faster.'

DoD adopted 'operations tempo' as a measure of the pace of an operation or operations in terms of equipment usage -- aircraft 'flying hours,' ship 'steaming days' or tank 'driving miles.' In the military way, the term became jargon: optempo."

Source: DefenseLINK at http://www.defenselink.mil/news/Aug1999/n08181999_9908181.html

So according to the largest 'user' of the term, it is a measurement of how fast you are using up the useful life of your military capital equipment - tanks, ships, aircraft - and is expressed in units peculiar to the service using the term. Since this is too narrow a definition to cover all the uses to which the phrase or word has been put, a flotilla of new jargon has arisen, including (but not limited to): perstempo, worktempo and deptempo. Don't be discouraged, however; Congress has instructed the DoD and the Military to issue an official and standard definition for these phrases and their kin through provisions placed in the Military annual spending plan.