langue d'o·ïl 
(doil, doi, dô-ēl)
(doil, doi, dô-ēl)n.
The Romance language of medieval France north of the Loire River, on which modern French is based.
[French, from Old French : langue, language; see LANGUAGE + de, of; see LANGUE D'OC + oil, yes (ultimately from Latin hoc ille (fēcit), he (did) this : hoc, this; see OCCITAN + ille, he; see al-1 in the Appendix of Indo-European roots).]