Phospholipid bilayer. Definition. noun. The two layers of phospholipids arranged in such a way that their hydrophobic tails are projecting inwards while their polar head groups are projecting on the outside surfaces. Supplement. This arrangement of phospholipids in bilayer is used to describe the cell membranes of many animal and plant cells. The two layers of phospholipids are arranged in a way that their hydrophobic tails are projecting to the interior whereas their hydrophilic heads are projecting the exterior.