IPA: /ˌæntiˈsɛpəˌrætɪst/
KK: /æntiˈsɛpəˌrætɪst/
A person who is against the idea of separating a group from a larger entity, often in a political or social context.
The antiseparatist movement gained momentum as more people voiced their opposition to division.
Antiseparatist is formed from "anti-" (meaning against) and "separatist" (from Latin "separare", meaning to separate, and the suffix "-ist", meaning a person who practices or advocates). The word refers to a person who is against separation or the act of separating.
Think of someone who is 'against' ('anti-') the idea of 'separating' ('separatist') — that's what an antiseparatist believes.