Relator Definition: An informer; a person who has supplied the facts required for a criminal prosecution or a civil suit. An informer; the person who has supplied the facts required for a criminal prosecution or a civil suit. In criminal prosecutions in some states, this would be indicated by the use of the expression ex. rel. as in The State of California ex. rel. Robert Smith v. George Doe. Find you are constantly looking up definitions? Try our search provider (works in most modern browsers) If you find an error or omission in Duhaime's Legal Dictionary, or if you have legal term suggestion, we'd love to hear from you!