1. Pertaining to rain, or more broadly, to precipitation, particularly to an abundant amount thereof. Compare hyetal, glacial. 2. Pertaining to an interval of geologic time that was marked by a relatively large amount of precipitation; the opposite of interpluvial. This is usually applied to those periods of heavy rainfall in the lower latitudes associated with the equatorward advance of the glacierization of an ice age. Thus, a pluvial period of low latitudes generally coincides with a glacial stage of higher latitudes. During pluvial periods the surfaces of existing lakes rose to high levels. In some cases several lakes combined into great sheets of water, such as Lakes Lahontan and Bonneville in the Great Basin of the western United States.