Although fluid is sometimes used as an alternative word for liquid, air is treated as a fluid in the field of fluid mechanics. Objects moving through air encounter forces and experience conditions similar to objects traveling through liquids.
Fluid refers to a substance through which atoms can flow freely. While there are differences between the ways liquids and gases behave, there are enough similarities to make this categorization useful. Plasmas are also considered fluids.
The field of fluid mechanics contrasts with that of solid mechanics. Not all substances, however, fall into one of these two categories. Rheology deals with these substances, which include sludges, muds and certain bodily fluids.