A bilayer refers to a structure composed of two layers of molecules, often seen in biological membranes. The most common example is the phospholipid bilayer, which forms the cell membrane. In this arrangement, the hydrophilic (water-attracting) heads of the phospholipids face outward, while the hydrophobic (water-repelling) tails point inward, creating a barrier that separates the interior of the cell from its external environment. Bilayers are crucial for maintaining the integrity of cells and regulating the movement of substances in and out. They provide a flexible yet stable environment for proteins and other molecules, facilitating essential processes such as cell signaling and transport.