A membrane is a thin, typically planar structure or material that separates two environments. Because it sits between environments or phases and has a finite volume, it can be referred to as an interphase rather than an interface. Membranes selectively control mass transport between the phases or environments.

Biological membranes include:

• plasma membrane and intracellular membranes
• mucous membrane

Artificial membranes are used in:

• Reverse osmosis
• Ultrafiltration
• Microfiltration
• Pervaporation
• Dialysis
• Electrodialysis
• Emulsion liquid membranes
• Membrane-based solvent extraction
• Membrane reactors
• Gas permeation
• supported liquid membranes