What is the basic definition of plasma?

The definition varies a bit from source to source, but here is one of them:

A plasma is an extremely hot gas that is composed of free-floating ions (atomic nuclei stripped of some electrons - making the ions positively charged) and free electrons (negatively charged). A plasma behaves much differently than a neutral gas, and is considered the fourth state of matter. A plasma conducts electrical currents. Stars are composed of plasma (source)

... or more simply:
A state of matter composed of charged particles, like ions and electrons, and possibly some neutral particles.

Good answer rick, I had to spread it though :)