Esperanto ( or ) is the world's most widely spoken constructed international auxiliary language. Created by the Warsaw-based ophthalmologist L. L. Zamenhof in 1887, it was intended to be a universal second language for international communication, or "the international language" (la Lingvo Internacia). Zamenhof first described the language in Dr. Esperanto's International Language (Esperanto: Unua Libro), 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".

Within the range of constructed languages, Esperanto occupies a middle ground between "naturalistic" (imitating existing natural languages) and a priori (where features are not based on existing languages). Esperanto's vocabulary, syntax and semantics derive predominantly from languages of the Indo-European group. The vocabulary derives primarily from Romance languages, with some of it influenced from Germanic languages. One of the language's most notable features is its extensive system of derivation, where prefixes and suffixes may be freely combined with roots to generate words, making it possible to communicate effectively with a smaller set of words.

Esperanto is the most successful constructed international auxiliary language, and the only such language with a sizeable population of native speakers, of which there are perhaps several thousand. Usage estimates are difficult, but two estimates put the number of people who know how to speak Esperanto at around 100,000. Concentration of speakers is highest in Europe, East Asia, and South America. Although no country has adopted Esperanto officially, Esperantujo ("Esperanto-land") is used as a name for the collection of places where it is spoken. The language has also gained a noticeable presence on the internet in recent years, as it became increasingly accessible on platforms such as Duolingo, Wikipedia, Amikumu and Google Translate. Esperanto speakers are often called "Esperantists" (Esperantistoj).