Nimbus is a general term referring to any clouds or cloud systems which generate precipitation. Sometimes the term nimbus is extended to include all clouds of a type which commonly produce rain, snow, hail, et cetera. Nimbus clouds are popularly known as rain clouds. Frequently, nimbus clouds are referred to by their genus names with a prefixed "nimbo-" or a suffixed "-nimbus". It should be noted, however, that "cumulonimbus" usually refers to a genus of cloud separate from cumlus clouds which need not be generating precipitation, and that cumulus clouds producing precipitation, such as a cumulus congestus might, are not considered cumulonimbus clouds.

Nimbus clouds were originally described in 1802 by Luke Howard as "a cloud in the act of condensation into rain, hail, or snow".