A(n) allows some substances to move freely into and out of a cell by simple diffusion while it will always exclude some substances from crossing. However, some substances that cannot cross by simple diffusion can pass through with the help of either active or passive transport proteins.

Selectively permeable membrane

Explanation:

A selectively permeable membrane  is a polymeric membrane that allows some substances like ions or molecules to move freely into and out of a cell by simple diffusion while it will always exclude some substances from crossing the membrane barrier. Occasionally, some substances are allowed to cross the membrane barrier by specialized processes like passive transport, active transport of facilitated diffusion.

It helps to form a barrier between a cell and its environment, unlike the cytosol.

A most common example is the phospholipid bilayer of the cell.