triple product is the product of a 📝plasma's density, temperature, and energy confinement time — the single figure of merit by which 📝fusion plasma performance is judged.
The term is best understood against its near-neighbor, the 📝Lawson criterion: the triple product is the criterion's modern, three-factor form. Lawson's original 1955 analysis fixed a temperature and bounded the product of density and confinement time; folding temperature in produces one number that captures all three requirements at once — enough fuel ions, hot enough to fuse, insulated well enough to keep their heat. Each factor covers a distinct failure mode: a plasma can be dense and hot but leak heat too fast, or superbly confined but too thin to react. The temperature that counts is the 📝ion temperature, and for a 📝D-T reaction plasma the 📝ignition threshold is a triple product of roughly 3 × 10²¹ 📝keV·s/m³, most easily met near 15 keV.
The metric's virtue is comparability across decades and machines. From the 1960s through the late 1990s the best achieved triple product doubled roughly every couple of years, a pace often compared to Moore's law, and the leading experiments now sit within about an order of magnitude of the 📝energy confinement time (τ_E)-limited ignition threshold — which is why the remaining gap, not the distance already covered, frames every serious fusion roadmap.