Any material or device that seals by compression. Common types are U-packing, V-packing, cup packing, and A-rings. Packing is also the soft rings that mechanical seals replace to stop leakage. Packing must leak because it works on the theory of a series of pressure drops to reduce the stuffing box pressure to the point where the leakage is acceptable. A minimum of five rings of packing is required to do this.
See also Piston Rod Packing; Piston Rod Wiper Packing; Shaft Packing; Wiper Packing.