The states of matter and their definitions are as follows:
1. Solid: Has a definite shape and a definite volume.
2. Liquid: Has no shape, but definite volume
3. Gas: No shape, no volume
4. Plasma: No shape, no volume, ionized gas.
A plasma refers to a hot ionized gas, that is made up of equal number of both positive and negative charges. The features of plasma are significantly different from that of neutral gases, so they are considered as the fourth state of matter.