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??????