>>149517847 (OP)
There are two (arguably three) criteria for a system of ionized particles to be considered a plasma:
Firstly, the system should be quasineutral, that is to say there should be approximately a similar number of positively charged particles in the system as negatively charged ones. This is necessary for the system to form stable sheathes and other equilibrium structures.
Second, there must be a sufficient number density of particles such that the system can exhibit collective or fluid-like behaviors over its characteristic scale lengths.
The third, debatable, criteria is that the plasma should be collisionless, that is to say the plasma frequency which describes the rate at which the system can respond to changes should be much faster than the frequency with which charged particles collide with each other or with any background neutral population. However there are many cases in nature and in artificial plasmas where this third point may not hold, especially in low-temperature and weakly ionized plasmas, like those used in many industrial applications.
