A product of density, temperature, and confinement time that determines how close a thermonuclear fusion research facility is to the conditions necessary for fusion ignition. Its value is independent of the method by which the plasma was created, i.e. whether it was magnetic or inertial confinement. The fusion reaction with the lowest (and therefore most achievable) triple product threshold is the fusion of deuterium and tritium. To successfully operate a fusion plant running a D-T reaction, we need to achieve a triple product of about 5 × 10²¹ m⁻³ keV s or greater.