NOUN (3)

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;
- Example: "the knots allowed no slippage"

The Collaborative International Dictionary of English v.0.48:

Slippage \Slip"page\, n.
   The act of slipping; also, the amount of slipping.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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"

Moby Thesaurus II by Grady Ward, 1.0:

58 Moby Thesaurus words for "slippage":
   avalanche, coast, comedown, debasement, decadence, decadency,
   declension, declination, decline, deformation, degeneracy,
   degenerateness, degeneration, degradation, demotion, depravation,
   depravedness, depreciation, derogation, descent, deterioration,
   devolution, downtrend, downturn, downward mobility, downward trend,
   drop, dying, ebb, effeteness, fading, failing, failure,
   failure of nerve, fall, falling-off, glide, glissade, glissando,
   involution, landslide, landslip, lapse, loss of tone, regression,
   retrocession, retrogradation, retrogression, sideslip, skid, slide,
   slip, slither, slump, snowslide, snowslip, subsidence, wane