The part of your bike found where your fork steerer passes through your frame.
Many different variations and diameters. Here are some of the headsets we have in stock.

Pro
Pro Headset Expansion Nut For Carbon Steerer Tubes, 25mm, 1 1/8 Inch
£24.99
Currently Unavailable
Pro
Pro Headset Expansion Nut For Carbon Steerer Tubes, 25mm, 1 1/4 Inch
£29.99
Currently Unavailable