A gas actually is a fluid as defined as a substance where the continuum assumption holds (the mean free path is much shorter than the characteristic length of the problem under observation). The mechanics of fluids is the last unsolved area of classical physics due to the fact that the governing Navier-Stokes equations are underdetermined and can only be solved explicitly for certain simple cases.

Fluid Mechanics defines a fluid as:
A substance which undergoes continuous deformation when subjected to a shear stress.

By this definition, both gases and liquids are considered fluids. A characteristic property of fluids is flow; a continuous and irrecoverable change in position of one part of the substance relative to another part caused by a shear stress. In contrast: in a solid internal shear forces are maintained when the substance is stressed, a solid undergoes no flow, and the object may recover its original shape after release. Note that under certain assumptions, a system consisting of fine (flowing) solid particles can be modeled as a fluid.

The resistance to deformation offered by a fluid under a shear stress is called fluid viscosity. This is an important parameter to categorize the various types of fluids, or fluid models (for Fluid Mechanics calculations). The simplest fluid model is the perfect or ideal fluid. This model corresponds to a hypothetical gas or liquid that offers no resistance to shear, and thus has zero viscosity (an inviscid fluid). This model is sometimes used for real fluids with low viscosity.

Real fluids with non-zero viscosity are categorized in Newtonian fluids and non-Newtonian fluids. A Newtonian fluid is a fluid with a constant viscosity, for a fixed static pressure and temperature. If the viscosity is a function of the shear stress (or equivalently, the shear rate), the fluid is called a non-Newtonian fluid. Non-Newtonian fluids are further categorized by their rheological properties. The following list shows a more detailed categorization.

1. Ideal Fluid (Perfect or inviscid fluid) - Hypothetical gas or liquid with zero viscosity
2. Newtonian Fluid - Fluid with a constant viscosity at a fixed temperature and pressure
3. Non-Newtonian Fluid - Viscosity is a function of shear stress
1. Time Independent - properties are independent of time under shear
1. Bingham-plastic fluid - Like a Newtonian fluid, this type of fluid has a linear relationship between shear stress and shear rate, but this function does not go through the origin (i.e.a finite shear stress is required to initiate flow). Water suspensions of rock, grains, or sewage are examples of Bingham Plastics.
2. Pseudoplastic fluid - These are the most common non-Newtonian fluids (ex: polymer solutions, pigments, various oils). The apparent viscosity decreases with increasing shear rate.
3. Dilatant fluid - These fluids show behavior opposite to pseudoplastic fluids. The apparent viscosity increases with increasing shear rate. Quicksand is a typical example of a dilatant fluid ("The more you move, the harder it gets").
2. Time Dependent - properties are dependent of time under shear
1. Thixotropic Fluid - The structure breaks down as a function of time under shear. The shear stress (and thus viscosity) decreases when the fluid is subjected to a constant shear rate. Some examples of thixotropic fluids are inks, paints, drilling muds, blood, and ketchup ("shake the bottle and it will flow").
2. Rheopectic fluid - These materials will set, or build up (i.e. increase in apparent viscosity) very rapidly, when shaken or tapped. Materials such as bentonite sols and gypsum in water show this behavior.
3. Viscoelastic fluids - These fluids exhibit elastic recovery from deformations which occur during flow (exhibits characteristics of a solid). Gelatin is an example of a viscoelastic fluid.

Flu"id (?), a. [L. fluidus, fr. fluere to flow: cf. F. fluide. See Fluent.]

Having particles which easily move and change their relative position without a separation of the mass, and which easily yield to pressure; capable of flowing; liquid or gaseous.

Flu"id, n.

A fluid substance; a body whose particles move easily among themselves.

Fluid is a generic term, including liquids and gases as species. Water, air, and steam are fluids. By analogy, the term is sometimes applied to electricity and magnetism, as in phrases electric fluid, magnetic fluid, though not strictly appropriate.

Fluid dram, or Fluid drachm, a measure of capacity equal to one eighth of a fluid ounce. -- Fluid ounce. (a) In the United States, a measure of capacity, in apothecaries' or wine measure, equal to one sixteenth of a pint or 29.57 cubic centimeters. This, for water, is about 1.04158 ounces avoirdupois, or 455.6 grains. (b) In England, a measure of capacity equal to the twentieth part of an imperial pint. For water, this is the weight of the avoirdupois ounce, or 437.5 grains. -- Fluids of the body. Physiol. The circulating blood and lymph, the chyle, the gastric, pancreatic, and intestinal juices, the saliva, bile, urine, aqueous humor, and muscle serum are the more important fluids of the body. The tissues themselves contain a large amount of combined water, so much, that an entire human body dried in vacuo with a very moderate degree of heat gives about 66 per cent of water. -- Burning fluid, Elastic fluid, Electric fluid, Magnetic fluid, etc. See under Burning, Elastic, etc.