WK is right on this, of course, but unimportantly so. A devaluation suggests a conscious act by a central bank BUT it's not hard-and-fast. For example, letting a currency depreciate as the Chinese did recently by not intervening to support it, can be described as a devaluation in effect or, less clearly, as an effective devaluation. Remainers often say that sterling has been effectively devalued. And, of course, context generally provides a very clear guide to what a chap actually means, whichever of the two words he uses.