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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: