WordNet records no synonym and no antonym for slippage. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a decrease of transmitted power in a mechanical system caused by slipping
decline from a standard level of performance or achievement
failing to hold or slipping out of place
“the knots allowed no slippage”
Synonyms, antonyms and the broader and narrower relations for slippage come from Princeton University’s WordNet. Full sources and licences.