Interfacing is a stiffening layer sewn or ironed between two pieces of cloth. Fusible interfacing, cut the interfacing, add interfacing to the collar.
It is what keeps a collar or a waistband from going limp, and it never shows on the finished garment. The fusible kind has glue on one side and is pressed on with an iron rather than stitched.
1nouninterfacingA stiffening layer sewn between two pieces of cloth.
“Cut the interfacing slightly smaller than the collar piece.”
interfacing is a material.
interface comes from inter- + face.
Etymology from Wiktionary, CC BY-SA