irreversible process
An irreversible process is any real change that cannot be undone by simply running it backward — the world is left permanently altered. Drop a coffee cup and it smashes; you will never see the shards leap back together. Stir milk into coffee and it mixes; it never unmixes on its own. Let a hot pan cool in a cold kitchen; the heat never spontaneously flows back to reheat the pan. Every process you actually witness in daily life is, to some degree, irreversible.
Physically, a process is irreversible whenever it involves friction, turbulence, unrestrained expansion, mixing, or heat flowing across a finite temperature difference. All of these generate entropy — the measure of disorder, or of energy spread out — so the total entropy of the system plus surroundings goes UP and never comes back down. That one-way increase is the content of the second law of thermodynamics, and it is what makes the process impossible to reverse without pouring in extra work from outside.
Irreversibility is not a flaw in our machines; it is a fundamental feature of the universe, and it is deeply tied to why time seems to flow in one direction. You can of course tidy a messy room or refreeze melted ice, but only by doing work and dumping even more entropy somewhere else, so the grand total still rises. The honest summary: local order can be bought, but only at the price of greater disorder overall.
Place a hot 80 C metal block against a cold 20 C one. Heat flows from hot to cold until both reach the same warm temperature. The reverse — the blocks spontaneously un-mixing back to 80 and 20 — never happens, because it would decrease the total entropy.
Heat flows hot-to-cold on its own but never cold-to-hot; that one-wayness is irreversibility.
Irreversible does not mean the system can never return to its old state — you can refreeze water — but you cannot do it without increasing the total entropy of the surroundings, so the universe as a whole never goes back.