A heat cycle is one pass of a race tire from cold up to operating temperature and back to cold. Each cycle permanently changes the tread compound, and racers disagree on what counts as one.
Tire Rack’s heat-cycling service defines the deliberate version tightly: run the tire up to temperature on rollers, watching tread temp with a pyrometer until it reaches 170-180°F, then let it sit a minimum of 24 hours so “the rubber bonds to relink in a more uniform manner.” Skip the rest and there is no benefit. That is not what most racers mean when counting cycles. A Grassroots Motorsports thread asking this exact question never lands on one answer — one poster separates the conditioning “initial heat cycle” from simply counting every track session as a cycle. Scuffing is a third thing: one light break-in run, and Tire Rack notes its heat-cycling service does not scuff in the tires. So when someone sells you a three-heat-cycle tire, ask what they counted.
Compound changes with every cycle, so what a used tire is worth depends on a count that nobody defines the same way.