Ptolemaïs (ptôlĭmĪsˈ) [key], town (1991 pop. 25,195), N Greece, in Macedonia. It was a small market town until 1958, when it began to be developed as an industrial center. Lignite, mined there in vast quantities, is used to power thermoelectric plants, which produce electricity for iron and steel mills, aluminum factories, and chemical plants. New dams in the area provide additional hydroelectric power.