In the year of likely around 966, Mieszko I united all of the tribes occupying a part of today's western Poland through Christianization of it, therefore making Poland an official sovereign state (at that time it was a Duchy though I did not find anything that would indicate that its official name was "Duchy of Poland").
26 years later, his son Bolesław I the Brave, became the new head of state and 59 years later transformed the country to a kingdom, crowning himself a king. From that day on the country during this period was officially called "Kingdom of Poland", but what was the official name of the country before that??????
From the Wikipedia article on Poland in the Early Middle Ages:
Under duke Mieszko I of the Piast dynasty, the expanded Polan territory was converted to Christianity in 966, which is generally regarded the birth of the Polish state. The contemporary names of the realm, "Mieszko's state" or "Gniezno state", were dropped soon afterwards in favour of "Poland" (or latinized as Polonia) , a rendering of the Polans' tribal name
The name "Poland" (Polska) comes from the name of the the tribe of western Polans - literally "people of the fields" ("pole" means both open area - plain - or an farming field) who became the dominant force around the 10th century in this region.
The first written notes about "Polonia" can be found around year 1000:
- John Canaprius,a Benedictine monk at the Aventine monastery in Rome wrote about Soběslav, the brother of Saint Adalbert of Prague who went cum Bolizlauo Palaniorum duce (with Boleslaw, prince of Poland)
- One of the songs about Saint Adalbert says Polania ergo tanti sepeliens floret martyryii pignora (uhhh… Poland buries the body of the martyr?)
- Annales Hildesheimenses from 1003 mention Heinricus Berthaldi comitis filius, et Bruno frater regis, et ambo Bolizavones, Polianicus vide licet ac Boemicus, a rege infideliter maiestatis rei deficient (Heinric, son of Berhold and Bruno, brother of the king and both Boleslaws - Polish and Czech left the circle of the friends of the emperor)