The minimum thickness of the lubricating oil between machine surfaces moving relative to one another. Hydrodynamic lubrication is the condition in which a stable film thickness results between a rotating journal and a stationary bearing. Film thickness is largely determined by the following three factors: the relative velocities of the moving surfaces, the base oil viscosity, and the magnitude of bearing load. The film thickness can also be influenced by the oil’s temperature and contamination. A healthy oil film is required for smooth operation and hydrodynamic bearing reliability.