From Urban Dictionary:

meme

(noun)

1. An idea, belief or belief system, or pattern of behavior that spreads throughout a culture either vertically by cultural inheritance (as by parents to children) or horizontally by cultural acquisition (as by peers, information media, and entertainment media).
2. A pervasive thought or thought pattern that replicates itself via cultural means; a parasitic code, a virus of the mind especially contagious to children and the impressionable.
3. The fundamental unit of information, analogous to the gene in emerging evolutionary theory of culture.
   • meme pool (n.) : all memes of a culture or individual
   • memetic (adj.) : relating to memes
   • memetics (n.) : the study of memes
4. In blogspeak, an idea that is spread from blog to blog.
5. An internet information generator, especially of random or contentless information.

Etymology : meme : derived from the Greek mimëma, ‘something imitated’, by Richard Dawkins in 1976