fusion cross-section is the effective target area a nucleus presents to an incoming particle for fusion, expressing the probability that a single collision at a given energy ends in a fusion reaction.
Cross-sections are measured in barns — 10^-28 square meters, nuclear physics' wry unit for a target "as big as a barn" — and fusion cross-sections are small by nuclear standards at the energies a confined plasma can reach. The value is not a fixed property of a fuel but varies steeply with collision energy: at low energy the 📝Coulomb barrier turns the nuclei aside, and the probability of tunneling through it rises rapidly as they collide harder. The 📝D-T reaction owes its primacy to this curve — its cross-section peaks near 5 barns at collision energies around 100 kiloelectronvolts, far above deuterium-deuterium fusion at the same energy, which is why first-generation power plants are designed around it.
The cross-section is a single-collision quantity, and its near-neighbor is the reactivity, written ⟨σv⟩ — the cross-section averaged, weighted by velocity, over the whole thermal distribution of a 📝plasma. Reactivity, not raw cross-section, sets a plasma's fusion power at a given temperature and density. The distinction matters because most fusion in a thermal plasma comes not from average ions but from the fast minority in the tail of the distribution, which the averaging captures and a single number read off the cross-section curve does not.