Damage to the cathode surface in a vacuum tube. Cathode stripping is not a literal “stripping” of the cathode surface, but rather bombardment of the surface with ions, which can physically damage the cathode coating. In some cases, an electron moving toward the tube’s anode strikes a gas molecule (the vacuum in a tube is never perfect — some gas remains or may slowly leak in), which can cause the release of an electron from the molecule. With the loss of an electron, the gas molecule now has a positive charge (it becomes an ion), and is attracted to the cathode and strikes it with considerable force. A contributor to this is said to be applying voltage to the tube before the cathode has warmed up.
Many tube experts doubt whether cathode stripping is even possible or ever occurs. Others believe that it can only happen with directly heated tubes. Still others believe that cathode stripping only happens at high voltages (typically above 10 kilovolts).