Ludwik Łazarz Zamenhof (pronounced /ˈzɑːmɨnhɒf/ in English; born Eliezer Samenhof, December 15, 1859 – April 14, 1917) was an ophthalmologist, philologist, and the inventor of Esperanto, a constructed language designed for international communication.

As of 1975, Esperanto was taught in 600 schools to 20,000 students per year; and there were about 100 journals and 7500 books written in Esperanto, including translations from 65 languages. In addition, it had by that time been used in more than 700 international conferences. As of 2000, per Cambridge Encyclopedia, it had somewhere between 1 and 15 million speakers, according to sources referenced.

Zamenhof was born on December 15, 1859 in the town of Białystok in Poland (then part of the Russian Empire) to parents of Lithuanian Jewish descent. He considered his native language to be his father's Russian (or perhaps Belarusian, which was not considered distinct from Russian at the time and which appears to have had a strong influence on Esperanto phonology), but also spoke his mother's Yiddish natively; as he grew older, he spoke more Polish, and that became the native language of his children. His father was a German teacher, and he also spoke that fluently, though not as comfortably as Yiddish. Later he learned French, Latin, Greek, Hebrew, and English, and had an interest in Italian, Spanish and Lithuanian.