WordNet records no synonym and no antonym for probation. 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 trial period during which your character and abilities are tested to see whether you are suitable for work or for membership
a trial period during which an offender has time to redeem himself or herself
(law) a way of dealing with offenders without imprisoning them; a defendant found guilty of a crime is released by the court without imprisonment subject to conditions imposed by the court
“probation is part of the sentencing process”
Synonyms, antonyms and the broader and narrower relations for probation come from Princeton University’s WordNet. Full sources and licences.