A<unk>  is defined as a substance that continually deforms (flows) under an applied shear stress regardless of how small the applied stress. All liquids and all gases are<unk> s.<unk> s are a subset of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids.
Liquids form a free surface (that is, a surface not created by the container) while gases do not. The distinction between solids and<unk> s is not entirely obvious. The distinction is made by evaluating the viscosity of the substance.  Silly Putty can be considered either a solid or a<unk> , depending on the time period over which it is observed.<unk> s display such properties as:
not resisting deformation, or resisting it only lightly (viscosity), and
the ability to flow (also described as the ability to take on the shape of the container).
These properties are typically a function of their inability to support a shear stress in static equilibrium.
Solids can be subjected to shear stresses, and to normal stresses - both compressive and tensile.  In contrast, ideal<unk> s can only be subjected to normal, compressive stress which is called pressure.  Real<unk> s display viscosity and so are capable of being subjected to low levels of shear stress.
In a solid, shear stress is a function of strain, but in a<unk> , shear stress is a function of rate of strain. A consequence of this behavior is Pascal's law which describes the role of pressure in characterizing a<unk> 's state.
Depending on the relationship between shear stress, and the rate of strain and its derivatives,<unk> s can be characterized as:
Newtonian<unk> s : where stress is directly proportional to rate of strain, and
Non-Newtonian<unk> s : where stress is proportional to rate of strain, its higher powers and derivatives.
The behavior of<unk> s can be described by the Navier-Stokes equations - a set of partial differential equations which are based on:
continuity (conservation of mass),
conservation of linear momentum
conservation of angular momentum
conservation of energy.
The study of<unk> s is<unk>  mechanics, which is subdivided into<unk>  dynamics and<unk>  statics depending on whether the<unk>  is in motion.
Fluid
Fluence
Colloid
Ionic liquid
Blood
Filter 
Contrast medium
Radionuclide