Fluid mechanics is a macroscopic science; i.e., it is concerned with characteristics that can be observed and measured on the laboratory scale. Even though we cannot resolve the motions of individual molecules by measurement in our laboratory, we recognize that it is the aggregation of properties of individual molecules that determines the behavior of a macroscopic element of fluid. Our measuring instruments average the behavior of a very large number of molecules in the vicinity of a point in a fluid, thereby determining the macroscopic fluid properties at that point. For example, suppose we could measure the velocities of all the molecules within a cube of air that is 1 micrometer () on a side, or in volume. Since there are about molecules of air in a cubic meter, this tiny volume would still contain more than molecules, a very large number. These molecules would be moving in all directions, and their average speed would be about meters per second (). But if the air is stationary, the average value of the vector sum of the molecular velocities, , is zero. On the other hand, if the air is moving slowly, this average vector velocity will equal that of the bulk motion that we observe. As long as there are a large enough number of molecules in the volume element of interest during the time period of measurement, this averaging process defines macroscopic properties of the fluid that can be ascribed to a point located at the center of the element. These properties are thereby defined as continuous functions of space and time throughout the fluid. It is these average properties that we observe in the laboratory when we measure the velocity of the fluid, or its pressure or density, with suitable instruments.

We usually categorize common fluids as either gas or liquid. A liter of gas, such as air, has much less mass than a liter of liquid, such as water. The molecules of a gas move about quickly, occasionally colliding with each other but are mostly unrestrained by intermolecular forces. In a liquid, on the other hand, the molecules are always interacting with each other, jiggling about but not moving very far before being repulsed by a neighbor. When a gas is compressed into a smaller volume, molecular collisions occur more frequently and a molecule spends a smaller fraction of its time in free flight. When compressed enough so that its molecules are as close to each other as those in a liquid, a gas is indistinguishable from a liquid. While the distinction between a liquid and its gaseous vapor is important thermodynamically, they both exhibit fluid behavior on a macroscopic scale.

The dominant property of fluids is their propensity to flow spontaneously within their containers. They have to be stored in containers to prevent their moving off to other locations, in contrast to solids which need not be so constrained. The volume of a liquid is conserved when it is moved from one storage vessel to another, but a gas always expands to fill its container since its molecules are free to move about. Fluids have no ability to hold a shape independent of their surroundings. This property of fluids is a direct consequence of the inability of the intermolecular forces to maintain an unchanging angular orientation of the molecules with respect to each other. Fluid molecules that are close to each other at one instant can move far apart with relative ease.

Many fluids are mixtures of several chemical species, such as air which is composed of nitrogen, oxygen and many trace species. Liquids can be solutions of solute species dissolved in a solvent, such as sea water. Mixtures like these still retain the characteristics of fluids, but have the added feature that the proportions of the constituents can be separately measured and may vary from point to point within the fluid.

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