The Declaration of Boulogne (Esperanto: Bulonja Deklaracio; full name: "Deklaracio pri la esenco de Esperantismo", declaration about the essence of Esperantism) was a document written by L. L. Zamenhof and endorsed by the attendees of the first World Congress of Esperanto in Boulogne-sur-Mer, France in 1905. It defined "Esperantism" as a movement to promote the widespread use of Esperanto as a supplement to natural languages in international and inter-ethnic contexts, not as a substitute for them within their proper realm. It declared that the Esperanto movement as such is politically and religiously neutral. It noted that Esperanto is in the public domain and anyone can use it however they like, as the creator of the language resigned his rights to it at the beginning. It said that the only obligatory authority for Esperanto speakers is the Fundamento de Esperanto (a collection of the early grammar, dictionary and sample text documents), which all speakers of the language are recommended to imitate for the sake of stability in the language.
Finally, it defined an "Esperantist" as someone who knows and uses the language Esperanto for any purpose - even if following the Esperanto rules of word formation "esperantisto" rather means "follower of esperantism" (esperantismo).
End of the first world congress of Esperanto in Boulogne-sur-Mer.
The Zamenhof and Michaux families at the first Esperanto Congress, Boulogne 1905.