irreversible

Irreversible is an adjective used to describe processes, actions, or states that cannot be reversed to their original condition simply by reversing the changes that produced them. In everyday use, an irreversible outcome typically requires additional input, energy, or a different sequence of steps to be undone, and the original pathway may be permanently lost.

In thermodynamics, irreversibility refers to processes that generate entropy and cannot be reversed without external intervention. Real processes are usually irreversible due to factors such as friction, heat transfer across finite temperature differences, inelastic deformation, chemical reactions, and mixing. By contrast, an idealized reversible process is a theoretical construct that would occur with infinitesimal driving forces and no net entropy production.

In chemistry and physics, many reactions are effectively irreversible under given conditions, meaning the reverse reaction is negligible or requires different conditions to proceed. Examples include combustion and rapid oxidation, where products are difficult to reconvert into reactants under the same circumstances. Other reactions reach a state of equilibrium where both directions occur, though the net composition remains constant.

In mathematics and computation, irreversibility can describe a function or transformation that is not invertible, as information is lost in the process. In computing, irreversible operations erase information and dissipate energy, a concept contrasted with reversible computing, where information preservation is pursued to reduce energy loss. The notion of irreversibility also informs discussions of the arrow of time across physics and philosophy.