A term coined initially by Steve Melia to describe road design that still allows through-access for walking and cycling, but removes it for motor traffic.
This can be achieved either by a straightforward physical closure with bollards (or other engineering), or by the use of opposed one-way streets (with exemptions for cycling), or simply by signs.
Once a road or street has been 'filtered', it remains accessible to motor vehicles, but is no longer usable as a through-route.
Public Health England specifically call for filtering on residential streets, to encourage active travel, and improve public health. (p.15).