What does the 97% storage efficiency of a flywheel represent?

[nwilkins259]

In my exploration of energy storage options i came across a statistic several times that given a vacuum and magnetic bearings, a flywheel can achieve 97% storage efficiency.

Whether its true or not, what does 97% represent? Does it imply that the energy will hold regardless of time and its a one time transfer loss or do you lose 3% per period of time.

If i find the source ill post the link.

 

[russ_watters]

It must assume a certain time period -- or perhaps that's over 0 time.