to remove unwanted substances from water, air etc by passing it through a special substance or piece of equipment:
The water in the tank is constantly filtered.
The ozone layer filters harmful UV rays from the sun.
2 [intransitive always + adverb/preposition]
if people filter somewhere, they move gradually to that place:
Chattering noisily, the crowd began to filter into the auditorium.
3 [intransitive always + adverb/preposition]
if news, information etc filters somewhere, people gradually hear about it from each other:
The news gradually filtered through from Bombay last night.
4 [intransitive always + adverb/preposition]C
if light or sound filters into a place, it can be seen or heard only slightly:
Moonlight filtered in through the frosted window.
The familiar notes of Beethoven's 'Für Elise' filtered from the bar.
5 [intransitive and transitive] British EnglishTTC
if traffic filters, or if a system filters it, cars can turn left or right while other vehicles going straight ahead must wait
filter something ↔ outphrasal verb
to remove something, using a filter:
The pump filters out mud.
to remove words, information etc that you do not need or want:
Net users can filter out unwanted emails with software.