If you drive through a tunnel, you’ll notice that your cell phone reception is diminished. When something is diminished, it’s been weakened or lessened.
Diminished is an adjective used when something has lost its power, prestige, or worth. The value of a car, for instance, is diminished over time by wear and tear. In some instances, diminished means "belittled," or made to seem like less in the face of something perceived as better or greater. The popularity of a laptop, for instance, will be diminished if a cheaper, more powerful model is released. Leaders and politicians who get involved in scandals often suffer diminished reputations.