Aftermost length of the propeller shaft, running through the stern tube to the propeller; subject to bearing wear and torsional fatigue.
Regulatory detail & full definition
Class survey item with mandatory periodic withdrawals (typically every 5 years for full inspection, 15 years for replacement on older designs). Modern oil-lubricated stern tubes extend intervals; water-lubricated (rubber) bearings are mandatory in some jurisdictions for environmental reasons.