How is pressure defined in a fluid?

Pressure in a fluid is defined as the force exerted by the fluid per unit area. This concept applies to both gases and liquids and is crucial in understanding fluid dynamics and hydrostatics. When a force is applied to a surface in contact with a fluid, that force is distributed across a specific area. The pressure is calculated using the formula:

[ \text{Pressure} = \frac{\text{Force}}{\text{Area}} ]

This definition indicates that pressure increases with either an increase in the force applied or a decrease in the area over which the force is applied. Thus, the correct answer emphasizes the relationship between force and area, characterizing pressure as a measure of how concentrated that force is on a given surface.

The other options do not accurately represent the definition of pressure. For instance, force times area describes the total force acting on a surface but does not provide a measure of pressure itself. Force divided by unit volume relates to density or pressure calculations in some contexts but does not define pressure. Volume per unit mass describes a different physical property, such as specific volume, which is not relevant to the concept of pressure in fluids.