a narrow walkway, especially one high above the surrounding area, used to provide access or allow workers to stand or move, as over the stage in a theater, outside the roadway of a bridge, along the top of a railroad car, etc.
1885, "long, narrow footway," from cat + walk; in ref. to such narrowness of passage one has to cross carefully, as a cat walks. Originally of ships and theatrical back-stages. Application to fashion show runways is mid-20c.