Although it is difficult to give an exact definition of a membrane, a more general definition could be: a selective barrier between two phases, the term "selective" being inherent to a membrane or a membrane process.  It should be noted this is a macroscopic definition while separation should be considered at the microscopic level.  To obtain a more informative understanding, membranes can simply be classified by nature into two categories: biological and synthetic membranes. 
   
A membrane filter is a thin, polymeric film or filament made up of millions of microscopic pores.  The size of these pores determines the nominal pore size rating of the membrane.  Membrane filters can be used in quantitative separation or the filtration of suspended matter from liquids and gases.   Membranes have many different properties that affect the performance of the filter in applications.

Often times membrane filters are classified according to their structures into several categories in terms of "pore size" (listed from smallest to the largest); namely, gas separation, dialysis/hemodialysis, reverse osmosis, ultrafiltration, and microporous membranes.

Microporous membrane is defined as a thin walled structure having an open spongy morphology of precisely controlled pore size typically ranging from 0.03 μm up to 10 μm in diameter.  In terms of membrane geometry, three types of microporous membranes are commercially available: flat sheet, hollow fiber, and tubular membrane.  The flat sheet geometry is the most popular and useful configuration among these three types.  Microporous membrane can be further divided into unsupported and supported variants.  
  
Microporous membranes are readily characterized with various techniques including scanning electron microscopy, bubble-point method, mercury intrusion porometry, and permeation measurements.  Among these methods, the first two are commonly accepted and used in industry due to the ease and convenience of testing procedures. 
    
Microporous membrane has been industrialized for many years in the U.S. and Europe and found widespread use in various fields such as pharmaceuticals, electronics, beverages, chemicals, environmental, and analytical for traditional filtration, purification, clarification, cold sterilization, concentration and venting.  In addition to traditional applications, microporous membrane has also been recognized in non-traditional applications attributed to its specific 3D pore structure and high surface area per unit weight uniqueness. These applications include drug-abuse tests, one-Northern blot, Southern blot, and battery separator.