IPA: /kæmˈpeɪnər/
KK: /kæmˈpeɪnər/
A person who actively works to support a cause, often in politics or social issues, by promoting ideas and encouraging others to participate.
The campaigner spoke passionately about the need for environmental protection.
Campaigner → It is formed from "campaign" (from Old French *campagne*, meaning open country or field) and "-er" (meaning a person who). A campaigner is a person who actively participates in a campaign, often in a political or social context.
Think of someone who is actively working in an 'open field' ('campagne') to promote a cause or idea — that's what a campaigner does.