"A well produced 'subvert' mimics the look and feel of the targeted ad, promoting the classic 'double take' as viewers suddenly realise they have been duped. Subverts create cognitive dissonance. It cuts through the hype and glitz of our mediated reality and, momentarily, reveals a deeper truth within." - Adbusters magazine 1999

Imagine walking down the street and seeing a billboard, let's say for...Shell. Shell don't really do much advertising these days, but just imagine. You walk closer...it's still a Shell advert to you, it still looks like one.

You become interested in this advert, so you walk in to get a closer look at the 200pt text along the bottom.

It's not a Shell advert.

That's not a Shell logo, that's a red and yellow cartoon skull uncannily shaped to look like the Shell logo. That's not a tagline extolling the virtues of Shell diesel, it's a statement about Shell's impact on the environment. You are left standing, suddenly...aware.

Congratulations. You've just been subvertised.
To put it in less imaginitive terms, to subvertise is to take an existing advertisement (in print, on a billboard, on TV) and change it ever so subtly so as to draw in J. Random Consumer, then smack him in the face with the metaphorical wet fish of an environmental/political message instead of "BUY OUR PRODUCT because WE SAY SO" (the element of surprise can not be doubted). As noted above, it is using The Man's tools against him.

subvertise.org and Adbusters are two sites that are well worth both your time and your donations if you're the slightest bit pissed off with "the system", or if you want to see some subvertisements in action.