fusion is the nuclear reaction in which two light atomic nuclei merge into a single heavier nucleus, converting a small fraction of their mass into a large release of energy — the process that powers the sun and every star.
The word comes from the Latin fusio, "a pouring or melting," and its everyday sense — a merging of distinct things into one — long predates the nuclear one, which entered physics in the 1930s as the mechanism behind stellar energy was worked out. Fusion is routinely confused with fission, its practical opposite: fission splits heavy nuclei like uranium and leaves long-lived radioactive waste, while fusion joins light nuclei like hydrogen isotopes, produces no chain reaction, and cannot melt down.
Forcing two positively charged nuclei close enough to fuse means overcoming their electrostatic repulsion, which requires temperatures above 100 million degrees Celsius — conditions under which matter exists only as 📝plasma. The engineering challenge of fusion energy is holding such a plasma hot, dense, and stable long enough for the energy released to exceed the energy spent, whether by magnetic confinement in a 📝Tokamak or by inertial compression with lasers. A power plant based on fusion would run on fuel derived from seawater and lithium, emit no carbon, and produce no long-lived waste.