From wikipedia:

Impermanence, also called Anicca or Anitya, is one of the essential doctrines and a part of three marks of existence in Buddhism. The doctrine asserts that all of conditioned existence, without exception, is "transient, evanescent, inconstant". All temporal things, whether material or mental, are compounded objects in a continuous change of condition, subject to decline and destruction. The concept of impermanence is also found in various schools of Hinduism and Jainism.

Anicca or impermanence is understood in Buddhism as the first of three marks of existence, the other two being dukkha (suffering, pain, unsatisfactoriness) and anatta (non-self, non-soul, no essence).

All physical and mental events, states Buddhism, come into being and dissolve. Human life embodies this flux in the aging process, the cycle of repeated birth and death (Samsara), nothing lasts, and everything decays. This is applicable to all beings and their environs, including beings who have reincarnated in deva (god) and naraka (hell) realms. This is in contrast to nirvana, the reality that is Nicca, or knows no change, decay or death.