Fluid

Continuum having the property of fluidity, i.e., permitting unbounded variation in shape under the action of arbitrarily small forces.
Comment. 1. In fluid mechanics and heat transfer the term "fluid" denotes both a "liquid" and a "gas."
2. If under the conditions being considered the change in density of the fluid with pressure is negliglibly small (compared to the density itself), it is considered to be an "incompressible fluid," and in the opposite case, as a "compressible fluid."