Esperanto is the world's most widely spoken constructed international auxiliary language. Created by L. L. Zamenhof in 1887, it is intended to be a universal second language for international communication, or "the international language". Zamenhof first described the language in Dr. Esperanto's International Language, which he published under the pseudonym Doktoro Esperanto. Early adopters of the language liked the name Esperanto and soon used it to describe his language. The word esperanto translates into English as "one who hopes".

Fundamento de Esperanto is a 1905 book by L. L. Zamenhof, in which the author explains the basic grammar rules and vocabulary that constitute the basis of the constructed language Esperanto. On August 9, 1905, it was made the only obligatory authority over the language by the Declaration of Boulogne at the first World Esperanto Congress. Much of the content of the book is a reproduction of content from Zamenhof's earlier works, particularly Unua Libro.