What part is this? Circular threading spool gear, about 10 mm diameter
This is a timing belt pulley.

Specifically, is a GT2 timing pulley, 2&nbsp;mm pitch (between teeth), 6&nbsp;mm wide. The drive diameter is measured by the number of teeth (16 in this case) , the shaft diameter (bore) is measured in mm.

The 'GT2' part refers to the tooth profile, some other examples are shown half way down this page.