'impermanent' (or, as abstract noun, aniccatā, 'impermanence') 

is the first of the three characteristics of existence (tilakkhana, q.v.). It is from the fact of impermanence that, in most texts, the other two characteristics, suffering (dukkha) and not-self (anattā), are derived (S.22. 15; Ud.IV. I)

"Impermanence of things is the rising, passing and changing of things, or the disappearance of things that have become or arisen. The meaning is that these things never persist in the same way, but that they are vanishing dissolving 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 external: All formations 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 (khandha, q.v.), the twelve personal and external sense bases (āyatana q.v.), etc. Only Nibbāna (q.v.), which is unconditioned and not a formation (asankhata), is permanent (nicca, dhuva).

The insight leading to the first stage of deliverance, Stream-entry (sotāpatti; s. ariya-puggala), is often expressed in terms of impermanence: "Whatever is subject to origination, is subject to cessation" (s. Dhammacakkappavattana Sutta, S.46. 11). In his last exhortation, before his Parinibbāna, the Buddha reminded his monks of the impermanence of existence as a spur to earnest effort: "Behold now, Bhikkhus, I exhort you: Formations are bound to vanish. Strive earnestly!" (vayadhammā sankhārā, appamādena sampādetha; D. 16).

Without the deep insight into the impermanence and insubstantiality of all phenomena of existence there is no attainment of deliverance. Hence comprehension of impermanence gained by direct meditative experience heads two lists of insight knowledge: 

• (a) contemplation of impermanence (aniccānupassanā) is the first of the 18 chief kinds of insight (q.v.); 

• (b) the 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' (s. visuddhi, VI). - 

Contemplation of impermanence leads to the conditionless deliverance (animitta-vimokkha; s. vimokkha). As herein the faculty of confidence (saddhindriya) is outstanding, he who attains in that way the path of Stream-entry is called a faith-devotee (saddhānusārī; s. 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)