Fluids are substances that can flow and conform to the shape of their containers. They include both liquids and gases.
Density is the mass per unit volume of a substance, typically measured in $\text{kg/m}^3$.
Pascal's Principle states that pressure applied to an enclosed fluid is transmitted undiminished to every part of the fluid and the walls of its container.
The buoyant force is the upward force exerted on an object submerged in a fluid, equal to the weight of the displaced fluid.