Thimble: a metal or plastic cap that is usually placed on the middle finger. A thimble protects the finger when hand sewing. It is most often used when tailoring suits or hand quilting.
Thimbles come in different sizes, so it is essential to try them on to check for fit. You're looking for a fit that is not to tight and not too loose. You should be able to feel the thimble wrap around your finger, but it should not feel uncomfortable.