WordNet records no synonym and no antonym for welterweight. 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 weight of 28 pounds; sometimes imposed as a handicap in a horse race (such as a steeplechase)
an amateur boxer who weighs no more than 148 pounds
a wrestler who weighs 154-172 pounds
Synonyms, antonyms and the broader and narrower relations for welterweight come from Princeton University’s WordNet. Full sources and licences.
a professional boxer who weighs between 141 and 147 pounds