A Molecular Sieve is a material containing tiny pores of a precise and uniform size that is used as an adsorbent for gases and liquids.
Molecules small enough to pass through are adsorbed while larger molecules are not. It is different from a common filter in that it operates on a molecular level. For instance, a water molecule may be small enough to pass through while larger molecules are not. Because of this, they often function as a desiccant. Molecular sieve can absorb water up to 22% of its own weight.

Often they consist of aluminosilicate minerals or synthetic compounds that have open structures through which small molecules can diffuse, such as clays, porous glasses, microporous charcoals, active carbons etc...