ˌopen ˈletter noun [countable] a letter to an important person, which is printed in a newspaper or magazine, usually in order to protest about something
[TahlilGaran] Dictionary of Contemporary English ▲
open letter n. phr. A public message in the form of a letter addressed to a particular person or to a group.
There was an open letter to the president of the United States in today's morning paper. [TahlilGaran] English Idioms Dictionary ▲