The term "cell membrane" refers to a thin, flexible barrier that surrounds and encloses the cell. It is composed mainly of proteins and lipids, which create a semi-permeable layer that separates the contents of the cell from its external environment. The primary function of this membrane is to regulate what enters and exits the cell by selectively allowing certain molecules or ions to pass through while preventing others from crossing. This process helps maintain the proper balance of nutrients, waste products, and other essential substances within the cell, which is crucial for its survival and functioning. In summary, "cell membranes" refers to the protective barrier that surrounds cells and plays a critical role in regulating their internal environment.