A fluid is a substance that can flow, spread and take the shape of a container
A compressible fluid is a fluid whose volume can change resulting is a change of pressure.
Gasses
A fluid with a fixed volume.
Liquids and Solids
Gas particles have no force of attraction between them so they take the available space. Liquid and solid particles have too much attraction between them so they won't change volume.
Pressure is force being applied in perpendicular motion
P=F/A A=F/P F=AxP