A fluid as hereafter considered is any material medium which cannot support shearing stresses and remain at rest, from the macroscopic standpoint. It a fluid is subjected to even the slightest shearing stress then motion must result.
Another way of saying the same thing is that the yield point of a fluid in shear is zero. Most liquids and gases fit in this category; in particular water and air.
Some real fluids possess finite yield points in tension and all can support finite compressive stresses in static equilibrium