WordNet records no synonym and no antonym for time constant. 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.
(electronics) the time required for the current or voltage in a circuit to rise or fall exponentially through approximately 63 per cent of its amplitude
the ratio of the inductance of a circuit in henries to its resistance in ohms
Synonyms, antonyms and the broader and narrower relations for time constant come from Princeton University’s WordNet. Full sources and licences.