To diminish means to become smaller, weaker, or less important, or to make something that way. When daylight diminishes in winter, the sun sets earlier each day and darkness arrives sooner. When a loud noise diminishes, it gradually gets quieter until you can barely hear it.

You can diminish something actively, like when you diminish a pile of homework by finishing one assignment at a time. A coach might try to diminish a player's nervousness before a big game by reminding them how well they've prepared. Sometimes people diminish other people's accomplishments by acting like they don't matter, which is unkind: saying “anybody could have done that” diminishes someone's real achievement.

The word often describes gradual change rather than sudden disappearance. Your enthusiasm for a new video game might diminish over time as you master all its levels. A flashlight's beam diminishes as its batteries wear down. Diminishing suggests something getting progressively smaller or weaker, like watching your shadow diminish as the sun rises higher in the sky.

Notice that diminish carries a slightly more serious tone than simple words like “shrink” or “lessen.” When scientists say a species' population is diminishing, they mean it's declining in a way that matters and deserves attention.