open letter
From Longman Dictionary of Contemporary EnglishRelated topics: Newspapers, printing, publishingopen letterˌopen ˈletter noun [countable] TCNPROTESTa letter to an important person, which is printed in a newspaper or magazine, usually in order to protest about somethingExamples from the Corpusopen letter• The open letter, in part a response to this development, galvanized that discussion.