Bobbin
/ˈbɑːbɪn/
Definition 1 of 3
Pro Tip 1/3Sewing Essential
Used specifically for sewing-related activities, often associated with machines. 
Make sure the bobbin is full before starting to sew a new garment.
Pro Tip 2/3Thread Holder
A bobbin holds the lower thread in a sewing machine and must be loaded correctly. 
If your stitching is loose, check if the bobbin is inserted properly.
Pro Tip 3/3Not Interchangeable
Bobbins come in different sizes; using the wrong one can damage your sewing machine. 
This sewing machine requires a specific bobbin size, not the one from my old machine.