plasma
/ PLAZ-muh /
Look at a lightning bolt, a neon sign, or the Sun, and you are looking at plasma. Heat a gas hot enough, or zap it hard enough, and its atoms start to come apart: electrons get stripped away from their nuclei. The result is a glowing, electrically charged soup that behaves quite unlike ordinary air.
Plasma is a gas in which a large fraction of the atoms have been ionised — that is, split into free negative electrons and positive ions. Because it is now full of loose charged particles, plasma conducts electricity, responds strongly to magnetic fields, and often glows as the electrons recombine and shed energy as light. It is frequently called the fourth state of matter, sitting beyond solid, liquid, and gas.
This matters because plasma is by far the most common state of ordinary matter in the universe — stars, the solar wind, and interstellar gas are all plasma — and on Earth it powers neon lights, plasma TVs, and fusion research. A common misconception is that plasma is exotic and rare; in fact you meet it daily in flames, sparks, and the bright pixel of a screen, and the cosmos is mostly made of it.
A neon sign is plasma on display: an electric current rips electrons off the neon atoms inside the tube, and as those atoms grab electrons back they release their energy as that familiar warm red-orange glow.
A neon sign glows because the gas inside has become plasma.
Plasma is not the same as the plasma in blood: in physics it means an ionised gas, a completely different meaning from the biological fluid that shares the name.