Promissory estoppel
A principle of equity that if you have made a promise, without consideration, but in circumstances that you know the other person relies on to its detriment, you can’t then go back on that promise, at least not retrospectively. You will be “estopped” (or, in modern language, “stopped”) from doing so. First articulated by Lance Cairns in Hughes v Metropolitan Railway, but resuscitated and given new life in the seventies by a spry young jurist by the name of Denning in the celebrated case of Central London Property Trust Ltd v High Trees House Ltd.