IPA: /ˌɒpəˈzɪʃənɪst/
KK: /ˌɑːpəˈzɪʃənɪst/
A person who is part of a group that opposes or disagrees with something, often in a political context.
The oppositionist voiced their concerns during the debate.
Oppositionist → It is formed from "oppositio" (from Latin, meaning a placing against) and "-ist" (meaning a person who practices or is concerned with). The word describes a person who is against or in opposition to something.
Think of someone who is 'placed against' ('oppositio') something — that's why an oppositionist is a person who opposes.