Nuclear fusion
Definition
The joining of two light atomic nuclei into a heavier one, releasing energy. This is the reaction that makes the Sun and all stars shine.
Unit: electronvolts (eV) or joules (J) for the energy released
Technical definition
A nuclear reaction in which two light nuclei overcome their electrostatic repulsion and merge into a heavier nucleus. The product mass is less than the combined reactant mass, and the difference is released as energy (E = mc^2). Fusion powers stars and thermonuclear weapons.
Example
In the Sun's core, hydrogen nuclei fuse into helium at about 15 million degrees Celsius. Every second the Sun converts roughly 600 million tons of hydrogen into helium, releasing the energy that warms Earth.
Fusion is not fission — fusion joins light nuclei (hydrogen into helium), fission splits heavy ones (uranium). And despite powering the Sun, controlled fusion on Earth remains an unsolved engineering challenge because it needs temperatures hotter than the Sun's core.