Question

what is fluid theory in plasma

Answer 1

In plasma fluid theory, a plasma is characterized by a few local parameters--such as the particle density, the kinetic temperature, and the flow velocity--the time evolution of which are determined by means of fluid equations. These equations are analogous to, but generally more complicated than, the equations of hydrodynamics.

Read more in lectures of Richard Fitzpatrick
https://farside.ph.utexas.edu/teaching/plasma/lectures/node27.html