Q (fusion energy gain factor) is the ratio of 📝fusion power produced by a plasma to the external heating power supplied to it — Q greater than 1 means net energy from the plasma.
The number's most common confusion is a question of boundaries. As usually quoted, Q is drawn at the edge of the 📝plasma: heating power in, fusion power out. It counts neither the electricity consumed turning wall-plug power into heating beams and waves nor the power that runs the rest of the facility, which is why a machine can pass Q of 1 while its host site still draws far more from the grid than it delivers — a distinction the field names Q_plasma vs Q_engineering.
The history of fusion research can be read as a climb up this one number. The Joint European Torus reached Q of 0.67 in 1997, the record for a 📝Tokamak; the National Ignition Facility exceeded Q of 1 with lasers in December 2022; ITER is designed for Q of 10, producing 500 megawatts of fusion power from 50 megawatts of heating. Q of 1 is called scientific breakeven, and a plasma that needs no external heating at all — ignition — corresponds to Q rising toward infinity.