The term for the structure that encloses and protects the cell is called the cell membrane, also known as the plasma membrane. This semi-permeable barrier regulates the movement of substances in and out of the cell, maintaining homeostasis. The cell membrane is composed largely of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates that contribute to its functionality. This structure is essential for communication with other cells, nutrient uptake, and the overall integrity of the cell.