A Hydrodynamic Bearing
is a bearing that uses a thin fluid layer as a way of keeping metal parts from touching, whilst allowing rotation.
In some ways it is similar to Hydrostatic Bearing except that no external pump is required. Instead, the rotation itself sweeps the lubricating fluid into the bearing and forms a tiny wedge under the bearing that stays there as long as the bearing turns and prevents contact.
Unlike Hydrostatic Bearings, a Hydrodynamic Bearing does in fact wear as the bearing starts up and stops; as the metal parts can touch until the fluid wedge forms.
The chief advantages of the Hydrodynamic Bearing is long life (provided the bearing is kept running most of the time), reliability, lack of required maintenance; and minimal variation in separation distance of the parts ('stiffness').