Real Dictionary

molecular sieve in American English
any of a class of zeolites or similar materials, natural or synthetic, having small, precisely uniform, pores in their crystal lattices that can absorb molecules small enough to pass through the pores: used as for separating or drying gases and liquids
noun: a compound with molecule-size pores, as some sodium aluminum silicates, that chemically locks molecules in them: used in purification and separation processes

molecular sieve in British English
noun: a material that can absorb large amounts of certain compounds while not absorbing others and is thus suitable for use in separating mixtures