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The original word base of Esperanto contained around 900 root words and was defined in Unua Libro ("First Book"), published by L. L. Zamenhof in 1887. In 1894, Zamenhof published the first Esperanto dictionary, Universala vortaro ("International Dictionary"), which was written in five languages and supplied a larger set of root words, adding 1740 new words.
The rules of the Esperanto language allow speakers to borrow words as needed, recommending only that they look for the most international words, and that they borrow one basic word and derive others from it, rather than borrowing many words with related meanings. Since then, many words have been borrowed from other languages, primarily those of Western Europe. In recent decades, most of the new borrowings or coinages have been technical or scientific terms; terms in everyday use are more likely to be derived from existing words (for example komputilo [a computer], from komputi [to compute]), or extending them to cover new meanings (for example muso [a mouse], now also signifies a computer input device, as in English). There are frequent debates among Esperanto speakers about whether a particular borrowing is justified, or whether the need can be met by derivation or extending the meaning of existing words.