Lingua Ignota (Latin for "unknown language") was described by the 12th century abbess of Rupertsberg, ildegard of Bingen, who apparently used it for mystical purposes. To write it, she used an alphabet of 23 letters, the litterae ignotae.
She partially described the language in a work titled Lingua Ignota per simplicem hominem Hildegardem prolata, which survived in two manuscripts, both dating to ca. 1200, the Wiesbaden Codex and a Berlin MS. The text is a glossary of 1011 words in Lingua Ignota, with glosses mostly in Latin, sometimes in German; the words appear to be a priori coinages, mostly nouns with a few adjectives. Grammatically it appears to be a partial relexification of Latin, that is, a language formed by substituting new vocabulary into an existing grammar.
The purpose of Lingua Ignota is unknown; nor do we know who besides its creator was familiar with it. In the 19th century some believed that Hildegard intended her language to be an ideal, universal language. However, nowadays it is generally assumed that Lingua Ignota was devised as a secret language; like Hildegard's "unheard music", it would have come to her by divine inspiration. Inasmuch as the language was constructed by Hildegard, it may be considered one of the earliest known constructed languages.
In a letter to Hildegard, her friend and provost Wolmarus, fearing that Hildegard would soon die, asks ubi tunc vox inauditae melodiae? et vox inauditae linguae? (Descemet, p. 346; "where, then, the voice of the unheard melody? And the voice of the unheard language?"), suggesting that the existence of Hildegard's language was known, but there were no initiates that would have preserved its knowledge after her death.
More information on the Wikipedia page [1].
For the musician see [2].