na-adjective
Describes something that holds a controlling or predominant position, such as an opinion, culture, or power structure.
その国では英語が支配的な言語だ。
English is the dominant language in that country.
会議では彼の
His opinion was dominant at the meeting.
優勢 means 'superiority' or 'predominance' in a competitive context, whereas 支配的 emphasizes a controlling or ruling influence.
Formed from 支配 (control, rule) + 的 (-like, -ive), a common pattern for creating adjectival nouns from nouns.