irreversibly

Irreversibly is an adverb used to describe an action, change, or process that cannot be reversed to its original condition. When something occurs irreversibly, the prior state cannot be restored by simply undoing the same steps. For example, burning wood transforms it into ash and gases, which cannot be returned to wood through any ordinary process.

The word derives from irreversibilis, meaning unable to be reversed, with the English suffix -ly forming an adverb. The related adjective irreversible refers to the property of being not reversible, whereas irreversibly specifies the manner of change. In technical writing, irreversibly is used to describe the direction of a process (e.g., "the system changes irreversibly") rather than to label a state.

In science, irreversible processes are a common concept in thermodynamics. The second law states that entropy tends to increase in isolated systems, making certain transitions effectively irreversible. Examples include free expansion of a gas, viscous dissipation, phase changes with heat transfer, and chemical reactions that proceed to completion. In theory, reversible processes are idealizations; practical processes exhibit irreversibility due to factors like friction, finite temperature differences, and mixing.

Outside science, irreversibly is used to emphasize permanent outcomes: irreversible damage, irreversible habitat loss, irreversible commitments. It is distinct from irrevocable, which often applies to decisions or promises that cannot be withdrawn.