What is the state of matter in a glowing fluorescent tube ?

The state of matter inside a glowing fluorescent tube is plasma. A fluorescent tube contains a gas (such as mercury vapor or argon) at low pressure. When an electric current passes through the tube, it provides energy that strips electrons from the gas atoms, creating a mixture of free electrons and positively charged ions. This ionized gas is known as plasma, often called the fourth state of matter.

The plasma itself emits ultraviolet (UV) radiation. This UV light then strikes the phosphor coating on the inner surface of the glass tube, which absorbs the energy and re-emits it as visible light (fluorescence). Other examples of plasma include neon signs, lightning, and the matter found in stars like the Sun.