Fluid flow described by spatial / temporal distribution of local properties: - velocity - pressure - temperature - etc. Closed vs open system - closed: mass inside system remains fixed - open: mass can flow across system boundaries The fluid velocity at a given point given by a vector field: $ \vec{V}=\vec{V}(x,y,z,t) $ Motion of a fluid element can be described as superposition of four components: 1. Translation 2. Rotation 3. Angular deformation 4. Volumetric deformation