Membranous glomerulonephritis: A disease affecting the glomerulus, the tiny structure in the kidney composed of capillary blood vessels that is actively involved in the filtration of the blood to form urine. The main sign of the disease is marked proteinuria. The majority of cases are primary, meaning the cause is not known. The remaining cases are secondary to conditions such as cancer, infection, and drug side effects. Membranous glomerulonephritis is a glomerular immune-complex disease, in which immune deposits form in the glomerular basement membrane, causing a membrane-like thickening. Also known as membranous nephropathy.