In the glomerulus of the kidney, the mesangium is a structure associated with the capillaries. It is continuous with the smooth muscles of the arterioles. It is outside the capillary lumen, but surrounded by capillaries. It is in the middle (meso) between the capillaries (angis). It is contained by the basement membrane, which surrounds both the capillaries and the mesangium.
This term is often used interchangeably with mesangial cell, but in this context refers specifically to the intraglomerular mesangial cells. These cells are phagocytic and secrete the amorphous basement membrane-like material known as the mesangial matrix. They are typically separated from the lumen of the capillaries by endothelial cells.