1. An outer cover: a shell casing; a tire casing.

2. The frame or framework for a window or door.

3. A metal pipe or tube used as a lining for a water, oil, or gas well.

4. A membranous case, often made of animal intestine, used to contain sausage or other processed meat.

5. A narrow passage for a rod or cord, as in a curtain or the waistband of a garment, made by folding over a small strip of fabric at its edge along its width and sewing it in place.