In the simplest definition, a line shaft is the rotating shaft part of a system of mechanical couplings between the power source and the machines that do work. Line shafts, also known as connecting shafts, jack shafts, or link shafts, are used to span long distances between rotating components.

Conventional solutions used to involve two standard flexible couplings and solid shafting, requiring intermediate pillow block bearings for support. Newer style precision line shafts can be operated at lengths of up to 6 m without any intermediate support required.