The plasma membrane is a membrane that surrounds the cell organelles or covers the cell and is hence known as the cell membrane.
It acts as a protective layer between the organelles of the cell and the external environment.
Also, the plasma membrane performs endocytosis and exocytosis.
The plasma membrane is technically called a selectively permeable membrane.
Because it only allows certain substances to move in and out of the cell.
Not every substance or any molecule can freely diffuse or move inside or outside the cell through the plasma membrane.
As the plasma membrane allows only the diffusion of some selective substances through its surface to go in and out of a cell, it can be also called as the selectively permeable membrane.