Anicca: Impermanent, transient or, as abstract noun, aniccatā impermanence or change is the first of the three universal characteristics of existence tilakkhana, which is easily observable and thus obvious. It is from this all-embracing fact of impermanence that the other two universal characteristics, suffering dukkha and no-self anattā, are derived see: S. XXII, 15; Ud. IV, I
impermanence of things is the arising, passing and changing of things, or the disappearance of things that have emerged & become into being. The meaning is that these things never persist in the same static state, but that they are changing, decaying, dissolving, and vanishing from moment to moment Vis.M VII, 3.
impermanence is a basic feature of all conditioned phenomena, be they material or mental, coarse or subtle, one’s own or other’s, internal or external: All these compounded constructions are impermanent sabbe sankhārā aniccā M. 35, Dhp. 277. That the totality of existence is impermanent is also often stated in terms of the five aggregates or clusters khandha, the twelve internal and external sense sources āyatana. Only Nibbāna, which is unconditioned and not a construction asankhata, is permanent, stable, still, static, lasting and constant nicca dhuva.
The insight leading to the first stage of deliverance: Stream-entry sotāpatti see: ariya-puggala, is often expressed in terms of impermanence: Whatever is subject to origination, is also subject to ceasing see: Dhammacakkappavattana Sutta, see: S. XLVI, 11. In his last exhortation, before his Parinibbāna, the Buddha reminded his Bhikkhus of the inevitable impermanence of all existence as a spur to earnest effort: Bhikkhus, I tell you: All constructions are bound to vanish. Strive enthusiasticly! Vayadhammā sankhārā, appamādena sampādetha; D. 16.
Without this deep insight into the impermanence and insubstantiality of all phenomena of existence there is no mental release, no relinquishment, no attainment of deliverance. Hence comprehension of impermanence gained by direct meditative experience heads two lists of insight knowledge: 1: Contemplation of impermanence aniccānupassanā is the first of the 18 chief kinds of insight, 2: Contemplation of arising and vanishing udayabbayānupassanā-ñāna is the first of 9 kinds of knowledge which lead to the purification by knowledge and vision of the path -progress see: visuddhi VI. Contemplation of impermanence leads to the signless deliverance animitta-vimokkha; see: vimokkha. As herein the ability of confidence saddhindriya is outstanding, he who attains in this way the path of Stream-entry is called a faith-devotee saddhānusārī see. ariya-puggala and at the seven higher stages he is called faith-liberated saddhā - vimutta, See also: anicca-saññā.
See The Three Basic Facts of Existence I: Impermanence WHEEL 186/187