The basolateral membrane of a polarized cell is the part of the plasma membrane that forms its basal and lateral surfaces, distinct from the apical (or lumenal) surface. This is particularly evident in epithelial cells, but also describes other polarized cells, such as neurons.
"Basolateral membrane" is a compound phrase referring to the terms basal (base) membrane and lateral (side) membrane, which, especially in epithelial cells, are essentially functionally identical in composition and activity. Proteins (such as ion channels and pumps) are free to move from the basal to the lateral surface of the cell or vice versa in accordance with the fluid mosaic model.
Tight junctions that join epithelial cells near their apical surface prevent the migration of proteins to the apical membrane. The basal and lateral surfaces thus remain roughly equivalent to one another, yet distinct from the apical surface.