Permeability refers to the ability of a surface to allow the transport of particles across it. A membrane is termed permeable if it allows the transport of molecules across it. A semi-permeable membrane is one that allows transport of only some molecules, while preventing the flow of others. An impermeable membrane does not allow the flow of any molecules across it. A semi-permeable membrane is also known as a selectively permeable membrane.
A great example of a semi-permeable membrane is the cellular membrane surrounding the cells that make up our body (and plants and animals, etc.). This membrane selectively allows certain molecules to enter the cell, while preventing the entry of all the other molecules. This ensures the safety of the cell and its sustained functioning.
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