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1.
A modified down tube for a water pipe that instead of just having one large hole opening into the water, instead the glass comes to a closed tip, along which are several smaller holes. This supposedly cools the smoke before being inhaled by diffusing (spreading) the smoke over a larger area.
Fuck, smashed my down tube cleaning it. Replaced it with a diffuser instead so my smoke should be cooler.
by jettt December 01, 2006
 
2.
A piece of automotive bodywork that is meant to deliver downforce by channeling air under the car, especially under the midsection and rear, so that it creates a low-pressure system under the car which sucks the vehicle to the ground. This increases traction and stability at high speeds.

These are especially prevalent on high-performance sports cars, supercars and most racing cars when putting spoilers that create similar downforce would just cause too much drag.
If you don't see a big tailwing on the back of a modern supercar, look at the tail for a diffuser.

The double-deck diffusers used by a few teams caused a scandal in Formula One over the 2009 season.
by Airrider October 10, 2009
 
3.
An emo-punk band. Most of their songs revolve around being hated by everyone. They sound like Linkin Park but with a twist of emo in it. I don't see how that's punk, honestly. Punk's about rising up against the system, doing what you feel like doing and not crying about your ex-girlfriend or being pushed around by everyone.

See Simple Plan.
Oh my God The World Hates Me
Oh My God My Girlfriend Left Me
There's nothing left in the world
I hope you're happy now
*A piss poor guitar solo*
by Iron Maiden November 04, 2004
 
4.
Great rock band, the album "Making the Grade" is an awesome album.
Diffuser is a great band.
by Wallis February 21, 2004