What is the outermost boundary of the cell called?

The plasma membrane is also known as the cell membrane and serves as the outermost boundary of the cell. The plasma membrane is a selectively permeable membrane that regulates the movement of substances in and out of the cell. It is composed of a lipid bilayer and various proteins that help to maintain the structural integrity of the membrane and facilitate the transport of substances across it. 

The cytoplasm is the gel-like substance inside the cell that contains all the cell's organelles and structures. 

The nuclear membrane is a double-membrane structure that surrounds the nucleus and separates it from the cytoplasm. 

The cytosol is the liquid portion of the cytoplasm, excluding the organelles.