Solids, for a certain shear stress, stop deforming at a certain shear strain (which is defined by an angular deformation). Fluids, on the other hand, are defined as a substance that deforms continuously under any shear stress. This shear is related proportionally to the rate of change of velocity through the height of a fluid element, with the proportionality constant of absolute viscosity.
$$ \tau=\mu\frac{du}{dy} $$
Another definitional property of liquids is the no-slip condition, which holds that the velocity of a liquid contacting a boundary is zero relative to the boundary, that their velocities are the same, they move together.
The relationship $\tau=\mu\frac{du}{dy}$ assumes a linear relationship between the shearing stress, $\tau$, and the rate of shearing strain, $\frac{du}{dy}$. Fluids that this is accurate for are Newtonian Fluids. Some fluids exhibit shear thinning, in which the viscosity decreases under shear strain (leading the slope of the stress vs. strain rate curve to gradually decrease), and others exhibit shear thickening, in which the viscosity increases under shear strain (leading the slope of the stress vs. strain rate curve to gradually increase).
Density, $\rho\>(\frac{kg}{m^3})\>or\>(\frac{slugs^*}{ft^3})$
$1\>slug=\frac{1\>lbf\>s^2}{ft}$
Specific Volume, $v=\frac 1\rho$
Specific Weight, $\gamma=\rho g\>(\frac{lbf}{ft^3})\>or\>(\frac{N}{m^3})$
Specific Gravity, $SG=\rho_{fluid}/\rho_{water@4\degree C}$
Typically we assume liquids to be incompressible, meaning that density is constant with respect to pressure.
On the other hand, we do not assume gases are incompressible. However, for small variations in elevation, we may still assume the density to be constant, as is explained later.