irreversible process
Stir a drop of milk into coffee and within seconds it is everywhere, swirled into a uniform tan. No amount of patient stirring will ever un-mix it back into a clean drop. That is an irreversible process: a change that runs naturally in one direction and cannot be undone without leaving some permanent mark on the world.
Technically, a process is irreversible if, after it happens, there is no way to return both the system and its surroundings to their exact starting states. Every real, finite-speed change is irreversible to some degree — friction, sudden expansions, mixing, heat crossing a real temperature gap. Each of these generates fresh entropy that did not exist before, entropy that cannot be un-generated.
Why it matters: irreversibility is where the abstract second law touches daily life. It is why engines waste fuel, why batteries run down, why we feel the arrow of time pointing forward. The key contrast: a reversible process is a frictionless ideal you can run backward step for step; an irreversible one always 'spends' something, producing entropy and falling short of the maximum work available. The honest note is that reversibility is the unreachable limit — everything that actually happens is irreversible.
Let a gas burst into a vacuum through an open valve (a 'free expansion'). It rushes to fill the space, doing no work and exchanging no heat — yet its entropy climbs, because the molecules now have far more room to roam. Nothing forces them back; this is irreversibility with no friction in sight.
Free expansion into a vacuum: entropy rises with no heat, no work, no friction.
Irreversible does not mean unrepeatable or un-fixable. You can always un-mix the milk by spending energy — but doing so dumps even more entropy elsewhere, so the universe's total never goes back down.