Invulnerability refers to the ability held by Kryptonians and other superbeings to be able to withstand incredible amounts of physical harm without taking any damage.
Invulnerability is the ability to be immune or highly resistant to one or more forms of physical damage or injury. Kryptonians have this power, among others, under the effects of the yellow sun. Some metahumans and other alien species also have varying degrees of invulnerability.
Kryptonians and Daxamites seem to gain this ability as a result of the Photonucleic Effect, that is, exposure to Earth's yellow sun. However, it is possible that they may have some degree of invulnerability even without the sun's effects; For example, Superman seems to have been operating largely without sun-granted powers in early issues of Action Comics, and was still able to withstand anything less than a bursting shell against his skin.