A KAS-4 rotary shaft seal is a specific type of dynamic seal used to prevent fluid leakage and contamination from external agents in rotating shafts, such as those in pumps and motors. This type of seal features a sealing lip made of rubber or another elastomeric material, which adheres to the rotating shaft, and a metal casing for stability.

Here is a more detailed definition:

• Seal: A mechanical component that seals the space between two parts of a machine — in this case, between the rotating shaft and its housing — preventing the passage of fluids or contaminants.
• Rotating shaft: A mechanical component that rotates, such as the shaft of a motor or a pump.

In summary, the KAS-4 seal is a gasket designed to seal rotating shafts, preventing fluid leaks and the ingress of external particles.