Essentially a word for "idea" but with overtones of natural selection applied to idea "survival."
The term was coined in Richard Dawkins' The Selfish Gene as an example of a replicator (the sound begs comparison with 'gene.') From there people made it into a comprehensive theory of everything. Susan Blackwell's book _The Meme Machine_ is a good example of some of the strengths and weaknesses in 'memetic theory.'