# What is meant by “collective behavior” in the definition of plasma?

"Plasmas are many-body systems, with enough mobile charged particles to cause some collective behavior ." [M.S. Murillo and J.C.Weisheit Physics Reports 302, 1-65 (1998)].

In the above definition what is meant by "collective behavior" ?

Plasmas are controlled by long-range interactions (i.e., Coulomb potentials and magnetic fields) and so the particles respond accordingly. Since electric fields do work to get rid of themselves, a plasma will reach what is called a quasi-neutral state, i.e., equal number densities of oppositely charged particles or: $$\sum_{s} \ Z_{s} \ n_{s} = 0$$ where $$Z_{s}$$ is the charge state of species $$s$$ and $$n_{s}$$ is the number density of species $$s$$.
This can manifest in a fluid-like behavior and the amount which the fluid-like behavior dominates depends upon the plasma parameters like number density, magnetic field, or plasma $$\beta$$ given by: $$\beta = \tfrac{ 2 \ \mu_{o} \ n_{o} \ k_{B} \ T }{ B_{o}^{2} }$$ where $$n_{o}$$ is the total number density, $$T$$ is the total temperature (see definition at https://physics.stackexchange.com/a/375611/59023), and $$B_{o}$$ is the magnetic field magnitude.
Highly collisional plasmas, e.g., solar photosphere, also behave like fluids as they are collisionally mediated. That is, the relevant frequencies like the cyclotron, $$\Omega_{cs}$$, and plasma, $$\omega_{ps}$$, frequencies are at or below particle-particle Coulomb collision frequencies (see definitions at https://physics.stackexchange.com/a/268594/59023).