intervener

/ˌɪntərˈvinər/
noun
  1. A person or group that becomes involved in a situation to change or influence it, especially in a conflict or dispute.
    • The teacher acted as an intervener when the students started arguing in class.
    • In the legal case, the judge allowed an intervener to present evidence on behalf of the environment.
    • The United Nations sent an intervener to help negotiate peace between the two countries.
Antonyms
What does "intervener" mean? | whatsthatwordmean | whatsthatwordmean