According to Wikipedia, 'nimbus' is "an outdated term for the type of cloud now classified as the nimbostratus cloud."
On its Nimbostratus cloud page, Wikipedia says that "in the 20th century, an IMC commission for the study of clouds put forward a refined and more restricted definition of the genus nimbus, effectively reclassifying it as a stratiform cloud type. It was then renamed nimbostratus, and published with the new name in the 1932 edition of the International Atlas of Clouds and of States of the Sky." It describes the appearance of nimbostratus clouds as "Dark and featureless layer cloud full of vapor, responsible for rain and snow."
© Haydn Thompson 2022