Esperanto

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An artificial international language with a vocabulary based on word roots common to many European languages and a regularized system of inflection.

from The Century Dictionary.

• noun The name of a recent ‘universal language’ constructed, like Volapük, by arbitrary reduction and manipulation of words and forms taken from existing European languages, and the adoption of a simple and regular inflection. The general aspect of the language as printed is that of a shrunken composite of Latin, Spanish, and French, with a Polynesian spelling.

from the GNU version of the Collaborative International Dictionary of English.

• noun An artificial language, intended to be universal, devised by Dr. Zamenhof, a Russian, who adopted the pseudonym “Dr. Esperanto” in publishing his first pamphlet regarding it in 1887. The vocabulary is very largely based upon words common to the chief European languages, and sounds peculiar to any one language are eliminated. The spelling is phonetic, and the accent (stress) is always on the penult. A revised and simplified form, called ido was developed in 1907, but Esperanto remained at the end of the 20th century the most popular aritficial language designed for normal human linguistic communication.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• proper noun The name of an international auxiliary language designed by L. L. Zamenhof with a base vocabulary inspired by Indo-European languages such as English, French, German, Italian, Spanish, and Russian, and having a streamlined grammar with completely regular conjugations, declensions, and inflections.
• proper noun figuratively Anything that is used as a single international medium in place of plural distinct national media.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an artificial language based as far as possible on words common to all the European languages