WordNet records no synonym and no antonym for critical mass. 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.
the minimum amount (of something) required to start or maintain a venture
“the battle for the computer market has now reached critical mass”
the minimum mass of fissionable material that can sustain a chain reaction
Synonyms, antonyms and the broader and narrower relations for critical mass come from Princeton University’s WordNet. Full sources and licences.