And what was their self-name before 966?
The Polans (also known as Polanes, Polanians; Polish: Polanie, derived from Old Slavic pole, "field" or "plain", see polje) were a West Slavic tribe, part of the Lechitic group, inhabiting the Warta River basin of the historic Greater Poland region in the 8th century.
In the 9th century the Polans united several West Slavic groups to the north of Great Moravia. The union led by the Piast dynasty developed into the Kingdom of Poland, whose name derives from that of the Polans.
en.wikipedia.org/wiki/Polans_(western)