A currency doesn’t just “weaken.” Someone with the authority to set its value decided it should be worth less than it was yesterday.
Devaluation is a government or central bank’s deliberate reduction of its currency’s official value against other currencies or a fixed peg (historically, gold) — distinct from depreciation, which describes a currency losing value through market trading rather than policy decision. Under a fixed exchange-rate regime, devaluation is usually a last resort: an admission that the peg can no longer be defended at its current level, typically under pressure from reserve depletion or an unsustainable trade position.
The effect redistributes wealth by design: holders of the currency and of fixed-value contracts denominated in it lose purchasing power overnight, while export-oriented producers and debtors owing in that currency effectively gain. Whether a devaluation is “correcting an imbalance” or “quietly transferring wealth from savers to the state” is largely a question of who benefits from each specific instance, not a property of devaluation as a mechanism.
Source: Magee, S.P., “Currency Contracts, Pass-Through, and Devaluation,” Brookings Papers on Economic Activity, 1973.