The part of the screen that recognises touch input. It's a transparent film with a ribbon cable that connects to the motherboard. The original nokia one in a nokia care store in Greece costs 60€ but you can find cheaper ones on ebay.

Beware though because I bought a relatively (for ebay prices) expensive one from an ebay seller for a 5800 because he claimed to sell an original one and it was not original (the original 5800 digitizer has a polarizing filter). Moreover it didn't work ok, the contacts weren't soldered correctly.