Source : Webster's Revised Unabridged Dictionary (1913)

Slippage \Slip"page\, n.
   The act of slipping; also, the amount of slipping.

Source : WordNet®

slippage
     n 1: a decrease of transmitted power in a mechanical system
          caused by slipping
     2: decline from a standard level of performance or achievement
     3: failing to hold or slipping out of place; "the knots allowed
        no slippage"