A grommet is a protective eyelet in a material that prevents tearing of either the material or rope passed through it. Also, described as a small ring of rubber that reinforces an eyelet.

One or two key dimensions dictate the choice of a grommet. This may be (A) inside diameter, (B) width of groove, (C) groove diameter, (D) outside diameter, (E) thickness. 
(These dimensions are illustrated above.)