IPA: /ˌɪntəˈviːnə/
KK: /ˌɪntərˈviːnər/
A person who takes part in a situation or legal case to help resolve it, often as a third party.
The intervenor presented new evidence to support the case.
Intervenor → It is formed from "inter-" (meaning between) and "venire" (meaning to come). An intervenor is someone who comes in between parties in a legal context to participate in a case.
Think of someone who 'comes in between' ('inter-') to join a legal matter — that's what an intervenor does.
No commonly confused words.