I. fil ‧ ter 1 /ˈfɪltə $ -ər/ BrE AmE noun [countable]
[ Date: 1300-1400 ; Language: Old French ; Origin: filtre 'piece of felt (= thick material) used as a filter' , from Medieval Latin filtrum ]
1 . something that you pass water, air etc through in order to remove unwanted substances and make it clean or suitable to use
water/air/oil etc filter
a pond filter
coffee filter papers
filter cigarettes (=with a filter at the end)
2 . a piece of equipment or computer program that only allows certain sounds, images, signals, types of information etc to pass through it:
a UVA light filter
The firm uses electronic filters to prevent workers from accessing the Internet.
3 . British English a traffic light that shows car drivers when they can turn right or left
II. filter 2 BrE AmE verb
1 . [transitive] 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] 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 English 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 ↔ out phrasal verb
1 . to remove something, using a filter:
The pump filters out mud.
2 . to remove words, information etc that you do not need or want:
Net users can filter out unwanted emails with software.