Low detail definition (real life)
Plasma is the 4th state of matter, beyond solid, liquid and gas. It is a collection of positively and negatively charged particles whose numbers are approximately equal.
Long description (real life)
A plasma is a very hot gas, where some or all of the atoms are separated into ions & electrons. As the negative and positive charges are free to move, plasma is an excellent conductor and is influenced by magnetic and electric fields. Most space plasmas have a very low density, for example the Solar Wind which averages only 10 particles per cubic-cm. Inter-particle collisions are unlikely - hence these plasmas are termed collisionless.