One of the Three characteristics of existence central to Buddhism. All experiences, perceptions, and forms arise and pass away; they are always in flux. This is true from the smallest sensation or concept to one’s existence itself.
The Buddhist claim is specifically that all sankhara are impermanent. That word’s meaning is subtle, and I don’t think I understand it well. Simple translations include “formations” and “conditioned things.” It’s meant to represent a fused “subject-object” epistemological thing. I think this means that Buddhism makes no claim that, say, the Platonic number 3 is impermanent: that’s just an object. But our thoughts, claims, and activities with respect to 3 are impermanent.
The Pali word for this is {anicca}.