The Full Form of STPC is Stopover Paid by Carrier.

A stopover is a long layover between flights that gives you enough time to explore the midpoint city outside the airport. You can build a stopover into your itinerary the old-fashioned way, by booking a multi-city itinerary, but some airlines make it even easier either by adding incentives to spend a day or two in the midpoint destination or by including an extra stop in the flight itinerary at no extra charge.

This free stopover is known as a “stopover paid by carrier,” or “STPC.”