Esperanto (/ˌɛspəˈrɑːntoʊ, -ˈræntoʊ/) is the world's most widely spoken constructed international auxiliary language. Created by L. L. Zamenhof in 1887 to be 'the International Language' (la Lingvo Internacia), it is intended to be a universal second language for international communication. He described the language in Dr. Esperanto's International Language (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 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. A substantial majority of its vocabulary (approximately 80%) derives from Romance languages, but it also contains elements derived from Germanic, Greek, and Slavic 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 (denaskuloj), of which there are an estimated 2,000. 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. It is becoming increasingly accessible on platforms such as Wikipedia, Amikumu, Google Translate and Duolingo. Esperanto speakers are often called Esperantists (Esperantistoj). A number of reforms, known as Esperantidos, have been proposed over the years.

Esperanto was created in the late 1870s and early 1880s by L. L. Zamenhof, a Jewish ophthalmologist from Białystok, then part of the Russian Empire, but now part of Poland. After several iterations (Proto-Esperanto), he self-published the first book of Esperanto grammar (Unua Libro) on July 26, 1887. He did so under the pseudonym Doktoro Esperanto (lit. 'one who hopes') and simply called the language "the international language" (la lingvo internacia). Early speakers grew fond of the name Esperanto and began to use it as the name for the language.

Zamenhof's goal was to create an easy and flexible language that would serve as a universal second language, to foster world peace and international understanding, and to build a "community of speakers". Zamenhof wrote that he wanted mankind to "learn and use ... en masse ... the proposed language as a living one". The goal for Esperanto to become an international auxiliary language was not Zamenhof's only goal; he also wanted to "enable the learner to make direct use of his knowledge with persons of any nationality, whether the language be universally accepted or not; in other words, the language is to be directly a means of international communication". His feelings and the situation in Białystok may be gleaned from an extract from his letter to Nikolai Borovko:

In Białystok the inhabitants were divided into four distinct elements: Russians, Poles, Germans, and Jews; each of these spoke their own language and looked on all the others as enemies. In such a town a sensitive nature feels more acutely than elsewhere the misery caused by language division and sees at every step that the diversity of languages is the first, or at least the most influential, basis for the separation of the human family into groups of enemies. I was brought up as an idealist; I was taught that all people were brothers, while outside in the street at every step I felt that there were no people, only Russians, Poles, Germans, Jews, and so on.

— L. L. Zamenhof, in a letter to Nikolai Borovko, c. 1895

Because people were reluctant to learn a new language which hardly anyone spoke, Zamenhof asked people to sign a promise to start learning Esperanto once ten million people made the same promise. He "was disappointed to receive only a thousand responses". Nevertheless, the number of speakers grew rapidly over the next few decades; at first, primarily in the Russian Empire and Central Europe, then in other parts of Europe, the Americas, China, and Japan.