intervenor

intransitive verb intervened, intervening

1. to come, be, or lie between
2. to take place between two events, points of time, etc.
3. to come or be in between as something unnecessary or irrelevant
4. to come between as an influence, as in order to modify, settle, or hinder some action, argument, etc.
5. Law to come in as a third party to a suit, to protect one's own interests