Irreversibility

n. quality of being irreversible, quality of being unchangeable

For the concept in evolutionary theory, see Dollo's law In thermodynamics, processes that are not reversible are termed irreversible. From this thermodynamics perspective, all natural processes are irreversible. The phenomenon of irreversibility results from the fact that if a thermodynamic system of interacting molecules is brought from one thermodynamic state to another, the configuration or arrangement of the atoms and molecules in the system will change as a result. A certain amount of "transformation energy" will be used as the molecules of the "working body" do work on each other when they change from one state to another. During this transformation, there will be a certain amount of heat energy loss or dissipation due to intermolecular friction and collisions; energy that will not be recoverable if the process is reversed.
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Noun
1. the quality of being irreversible (once done it cannot be changed)
(antonym) reversibility
(hypernym) changelessness, unchangeability, unchangeableness, unchangingness

(n.)
The state or quality of being irreversible; irreversibleness.
  

Irreversibility, Law of In biology, the concept that "in evolution no entity, losing an organ or a character or a feature, takes that identical organ up again, or regains it; but that if the recurrent conditions of environment are ever similar to what they were before, he then gains new organs suited to these recurrent conditions in the new circumstances in which he finds himself" {MIE 102}.