A filter is a device (usually a membrane or layer) that is designed to physically block certain objects or substances while letting others through. Filters are often used to remove solid substances suspended in fluids, for example to remove air pollution, to make water drinkable. Some devices that are called filters may also carry out other processes, such as waste treatment, (e.g. biofilter).

The process of passing a mixture through a filter is called filtration. The liquid produced after filtering a suspension of a solid in a liquid is called filtrate, while the solid remaining in the filter is called retentate or residue.