Super Strength is the ability amongst Kryptonians, Daxamites, and many other superbeings of having incredible strength. In the case of Kryptonians and Daxamites, it has been explained as a result of the planet Krypton having significantly greater gravity than Earth, leading its inhabitants to have significantly greater muscle mass. However, the Photonucleic Effect at the very least greatly increases this incredible strength. In some cases, it has been given as the explanation of this strength in general.
In the case of Doomsday, the photonucleic effect is almost certainly not in effect, and his strength is his own.