Domnitor (pl. domnitori) was the official title of the ruler of the United Principalities of Wallachia and Moldavia between 1859 and 1866. It was usually translated as prince in other languages. Derived from the Romanian word "domn" (lord or ruler) and, in turn, from the Latin "Dominus", Domnitor had been in use since the Middle Ages. Wallachian and Moldavian rulers had sometimes been referred to by the term, though their official titles had been voievod or hospodar, especially after they were officially nominated by the Sultans of the Ottoman Empire. The title acquired an officially recognized meaning only after the 1862 union of Wallachia and Moldavia under Alexandru Ioan Cuza. The title was held by Cuza and his successor, Carol I, until the proclamation of the Kingdom of Romania in 1881.