What's the main characteristic of a nimbus cloud?
And the answer: precipitation.
Adding "nimbus" to a cloud name means precipitation is falling from the cloud. For example, if a cumulus cloud produces rain, it's known as a cumulonimbus cloud.
While there are many types of clouds in the sky, only certain types of clouds can produce rain. For one, nimbostratus clouds are low-hanging rain clouds that tend to be what we think of when we consider an overcast day. These clouds usually indicate strengthening and thickening stratus layer, and usually tend to produce light rain.
But cumulonimbus clouds are the main rain producers. These clouds are large and puffy, and though they might hang low, they tend to extend high into the atmosphere. These types of clouds produce strong and intermittent precipitation and tend to be associated with more severe weather. More intense cumulonimbus clouds can produce wind, lightning, thunder, and even tornadoes. Cumulus congestus clouds, their smaller sibling, can also produce rain, but tend not to be significant enough to merit a name change to "nimbus."
Learn more about clouds and the water cycle here.