A glass downstem with drilled or laser-cut holes at the bottom in order to increase bubbling to further cool the smoke. Used in a bong.
John: I got an ash-catch for my bong, and a new downstem.
Bill: Is it a diffused downstem?
John: Of course, you think I'm crazed?
The female part of a bong's downstem that you put the actual bowlpiece into that specifically has around 10-15 small holes blown near the end of the tube that goes into the water.
Contrary to what the dipshit who posted the first definition thought, these holes are not drilled or cut with a laser. I don't know where he got his information, but i have made many diffused downstems in my day, and i certainly do not own a laser or a glass drill bit. I own a torch, and the holes ARE BLOWN LIKE ALL OTHER FORMS OF GLASSWARE INTENDED FOR SMOKING PURPOSES!!!! What a shocker, huh? That glass blowing would involve blowing. The science behind it is that you crank up the oxygen in a oxygen/propane torch so that the flame becomes concentrated, and ultra-hot. Then you heat up a small area that you want one of the holes to cover, and then blow into the tube until the area bubbles up, then pops like gum. You repeat this until there are a sufficient number of holes for there to be little to no drag when hitting the bong that the downstem is intended for. Used properly, this form of downstem enables the smoker to take bigger hits as the smoke is cooled and filtered more efficiently.
The only time a laser is used is for slitted diffusion openings, not holes. This is only found in very upscale bongs like ROORs.
Joe: Hey man, i got 80 bucks, can you make me a kickass bong?
Tom: Yeah for sure, any specific requirements?
Joe: Yeah make that shit with a diffused downstem, i like to take fatty rips of the cheeba.