re ‧ form ‧ er /rɪˈfɔːmə $ -ɔːrmər/ BrE AmE noun [countable]
[ Word Family: noun : ↑ form , ↑ formation , ↑ transformation , ↑ reformer , ↑ reform , ↑ reformation , ↑ reformist , ↑ transformer , ↑ formlessness ; verb : ↑ form , ↑ reform , ↑ transform ; adjective : reformed, ↑ reformist , ↑ formless ; adverb : ↑ formlessly ]
someone who works to improve a social or political system:
a great social reformer