Sentence:  The first practical <WSD>constructed language</WSD> was the south-west German Pastor Schleyer's Volapük from 1879; its complicated grammar and irregular vocabulary made learning difficult, however. The most successful has been Esperanto, devised by the Warsaw ophthalmologist Ludwig Zamenhof in 1887, that today can count some one million speakers. 
Meaning of the word marked with <WSD> tags:  A human language that has been consciously devised by an individual or a small group, as opposed to having naturally evolved as part of a culture like a natural language.