A layover is a short connection between flights at an intermediate airport—generally under 4 hours on domestic U.S. itineraries and under 24 hours on international ones. You are mainly waiting to change planes, not treating the connecting city as a destination.
A stopover is a longer, planned break that crosses those thresholds (4+ hours domestic, 24+ hours international). Airlines and award programs often price or ticket stopovers differently, and travelers usually leave the airport to stay overnight or explore the city before continuing.