1 a [+ object] : to pass (something, such as a gas or liquid) through a filter to remove something unwanted
They've begun filtering their water to remove impurities.
1 b [+ object] : to remove (something unwanted) by using a filter
a device that filters impurities from water
sunglasses that filter ultraviolet light
— often + out
sunglasses that filter out ultraviolet light
— often used figuratively
The purpose of the first interview is to filter out people who are not good candidates for the job.
2 a always followed by an adverb or preposition [no object] : to move through or into something in small amounts or in a gradual way
Sunlight filtered through the leaves.
His ideas have filtered down to his children.
2 b always followed by an adverb or preposition [no object] : to come or go slowly in small groups or amounts
The crowd filtered into the arena.
The crowd filtered out (of the arena).
Early election returns have begun to filter in.