Artificial language created in 1887 by Polish oculist Ludwik Lejzer Zamenhof, and intended for use as an international second language.

Zamenhof's Fundamento de Esperanto, published in 1905, give the introduction of the basic principles of the language's structure and formation.

The vocabulary is created from root words taken from Latin, Romance, and Germanic languages.

The number of Esperanto speakers is estimated at more than one hundred thousand.

More than 30,000 books have been published in Esperanto.

A declaration endorsed by the Esperanto movement in 1905 limits changes to Esperanto principle. That declaration stated, amongst other things, that the basis of the language should remain Fundamento de Esperanto ("Basis of Esperanto", a work by Zamenhof), which is to be binding forever: nobody has the right to make changes to it. The declaration also permits new concepts to be expressed as the speaker sees fit, but it recommends doing so in accordance with the original style. However, modern Esperanto usage may in fact depart from that originally described in the Fundamento. The translation given for "I like this one", in the phrases below offers a significant example. According to the Fundamento, Mi ŝatas ĉi tiun would in fact have meant "I esteem this one". The traditional usage would instead have been Ĉi tiu plaĉas al mi (literally, "this one is pleasing to me"), which, although it differs from the English phrasing in "I like this one", more closely reflects the phrasing in several other languages (e.g. French celui-ci me plaît, Spanish éste me gusta, Russian это мне нравится [eto mnye nravitsya], German Dieses gefällt mir).

Esperantists have formed many words to express concepts which have arisen more recently, but where possible these have indeed conformed to the existing style of the language. For example, "computer" is komputilo, (using the suffix -il- meaning a tool). Eŭro (as in the above phrases) is another good example: even though the currency is called Euro in all the European Community's official languages which use a Latin script, in Esperanto Eŭro was chosen because it better fits the pattern of the language.

Esperanto is not an official language of any country. However, it is the official working language of several non-profit organizations, mostly Esperanto organizations.

No new languages or dialects have formed through fragmentation of Esperanto as they do in natural languages, due mainly to the regular nature of the language and its intended field of use. People tend to create slang forms and regional variants in the language(s) they use day to day, rather than those used primarily for intercommunication with different-language speakers; in the case of Esperanto, such variations, if heavily different from the official Fundamento version, would make universal comprehension less likely and negate the intended purpose of the language.

There is also evidence that studying Esperanto before studying any other second language (especially an Indo-European language) speeds and improves learning, because learning subsequent foreign languages is easier than learning one's first, while the use of a grammatically simple auxiliary language lessens the "first foreign language" learning hurdle. In one study (Williams 1965), a group of high school students studied Esperanto for one year, then French for three years, and ended up with a better command of French than the control group, who studied French without Esperanto during all four years. However, the study failed to prove that Esperanto was responsible for this advantage specifically. It is likely that learning any language will benefit the future study of other languages.