1. Providing the means for two or more pieces of equipment or person to communicate or control the other. USB to DMX interface, for example. "Let's call a meeting so we can interface this"--argh! 2. Costumes. Stiff material used to reinforce, stabilize, or give body to a garment. For more, see Before You Choose Interfacing for Your Sewing Needs. The word "innerfacing" (which should be two words, meaning "facing inwards" or "the inside one" ) is often mistakenly used.