A meme hack is altering a meme to express a point of view not intended or inherent in the original, or even opposite to the original. The meme can be thoughts, concepts, ideas, theories, opinions, beliefs, practices, habits, songs, or icons. Distortions of corporate logos are also referred to as Subvertising.
From Samizdata: "Intentionally altering a concept or phrase, or using it in a different context, so as to subvert the meaning."
See also Stephen Downes, Hacking Memes (First Monday, volume 4, issue 11) for a detailed description of meme hacks.
Meme hacking is also referred to as subvertising or subliminal messaging.