Fluid mechanics is the subdiscipline of continuum mechanics that studies fluids, that is, liquids and gases.
It can be further subdivided into fluid statics, the study of fluids at rest, and fluid dynamics, the study of fluids in motion.
Fluids are composed of molecules that collide with one another and solid objects.
The continuum assumption, however, considers fluids to be continuous.
That is, properties such as density, pressure, temperature, and velocity are taken to be well-defined at infinitely small points, and are assumed to vary continuously from one point to another.
The fact that the fluid is made up of discrete molecules is ignored.