State of matter is a scientific term used to describe the physical form of a substance. PLASMA is a state of matter composed of charged particles, such as ions and electrons, and is often referred to as the fourth state of matter. GAS is a state of matter that has no definite shape or volume and is composed of molecules that are in constant motion. SOLID is a state of matter that has a definite shape and volume and is composed of particles that are closely packed together.