retentate

This term is used exclusively within technical, scientific, and industrial contexts, specifically in chemistry, pharmacology, and wastewater treatment. It describes the concentrated stream remaining after a membrane filtration process, such as reverse osmosis or ultrafiltration. It is the direct antonym of permeate, which refers to the liquid that successfully passes through the filter.

Because it describes a specific physical substance in a laboratory or industrial setting, it is treated as a mass noun when referring to the material in general, though it can be used as a countable noun when referring to specific batches or types of retained material.