fluid

Any substance, either liquid or gas, in which the molecules are relatively mobile and can ‘flow’. A fluid can be ‘pushed’ by applying a force that is transmitted to different parts of the fluid.

The pressure of a fluid increases with the depth of the fluid. In water, an upward force called the upthrust is the liquid pressure that balances the weight of an object in the water. As liquids can transmit pressure, they are used in hydraulic systems.