From Imperial Wiki
A meme is a single idea or unit of cultural information, the building block of a culture and its evolution. Most memes self-propagate virally and sometimes mutate as any virus would. Well-known examples of self-propagating memes are Chain Letters and many religions. Memes were first proposed by Professor Richard Dawkins.
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In common usage, a meme is a pop-culture image with a superimposed humorous statement.