The branch of Mechanics that deals with the motion and the forces that affect fluids

A fluid is a piece of matter that has no, or negligible, elasticity. This means it flows under virtually any applied force.

There are three main phases of matter that are fluid:

• Liquid. Fully homogeneous/isotropic condensed phase composed of atoms.
• Gas. Fully homogeneous/isotropic non-condensed phase composed of atoms.
• Plasma. Fully homogeneous/isotropic non-condensed phase composed of ionized atoms and/or other charged particles. See Plasma physics.

More complex fluid phases, often composed of mixtures are called complex fluids.

Fluid dynamics describes the causes of motion, i.e. the forces and torques that can affect fluids, and how these affect their motion. Magnetohydrodynamics and Electrohydrodynamics describe the dynamics of an electrically conductive fluid.

Fluid kinematics (just like kinematics, in Mechanics) describes the possible motion of fluids.