Plasma is just ionized gas, in other words, a gas of nuclei and electrons instead of a gas of atoms and molecules. One place I found on the web is at NASA's Space Academy web site . It was known that gas could be ionized long before the word plasma came into use. It is relatively easy to create it. It is even done in arc lights and in the novelty spheres that look like balls of lightning.
Lighting also causes plasma in the atmosphere.