the universe's receipt policy: some processes let you return the item, most don't.
means a property of a process where you can run it backward and land exactly where you started, with nothing lost or left over.
from formalized in 19th-century thermodynamics, when physicists studying steam engines needed a word for an idealized process with zero entropy production, distinguishing it from every real engine, which leaks a little disorder into the universe on every cycle.
carnot engine — 1824 thought experiment, the reversible ideal no real engine beats
chemical equilibrium — reactions like nitrogen dioxide dimerization flip direction with pressure
time reversal symmetry — newton's laws work identically if you run the film backward
scrambled egg — the classic irreversible process everyone actually understands