Anicca is a Pali word for "impermanent." The Buddha taught that everything conditioned is impermanent. To be conditioned is to be dependent on or affected by something else, and Buddhism teaches that all phenomena, including beings, are conditioned.
The transitory nature of all conditioned things is the basis of life. Because all phenomena are in a state of flux, change is possible. New life is possible, and enlightenment is possible, because of anicca.
Anicca, dukkha and anatta are considered to be the Three Marks of Existence by all Buddhists. In Mahayana Buddhism, because all phenomena are impermanent, and in a state of flux, they are understood to be empty of an intrinsic self (shunyata).