An open letter is a letter that is intended to be read by a wide audience, or a letter intended for an individual, but that is nonetheless widely distributed intentionally.
Open letters usually take the form of a letter addressed to an individual but provided to the public through newspapers and other media, such as a letter to the editor or blog. Especially common are critical open letters addressed to political leaders.
Letters patent are another form of open letter in which a legal document is both mailed to a person by a government and publicized so that all are made aware of it. Open letters can also be addressed directly to a group rather than any individual.
Two of the most famous and influential open letters are J'accuse...! by Émile Zola to the President of France, accusing the French government of wrongfully convicting Alfred Dreyfus for alleged espionage, and Martin Luther King Jr.'s Letter from Birmingham Jail, including the famous quotation "Injustice anywhere is a threat to justice everywhere".