Meme: an information pattern, held in an individual's memory, which is capable of being copied to another individual's memory. Memetics: the theoretical and empirical science that studies the replication, spread and evolution of memes.

Cultural evolution, including the evolution of knowledge, can be modelled through the same basic principles of variation and selection that underly biological evolution. This implies a shift from genes as units of biological information to a new type of units of cultural information: memes.

A meme is a cognitive or behavioral pattern that can be transmitted from one individual to another one. Since the individual who transmitted the meme will continue to carry it, the transmission can be interpreted as a replication: a copy of the meme is made in the memory of another individual, making him or her into a carrier of the meme. This process of self-reproduction (the memetic life-cycle), leading to spreading over a growing group of individuals, defines the meme as a replicator, similar in that respect to the gene (Dawkins, 1976; Moritz, 1991).
Dawkins listed the following three characteristics for any successful replicator:

copying-fidelity:
the more faithful the copy, the more will remain of the initial pattern after several rounds of copying. If a painting is reproduced by making photocopies from photocopies, the underlying pattern will quickly become unrecognizable.

fecundity:
the faster the rate of copying, the more the replicator will spread. An industrial printing press can churn out many more copies of a text than an office copying machine.

longevity:
the longer any instance of the replicating pattern survives, the more copies can be made of it. A drawing made by etching lines in the sand is likely to be erased before anybody could have photographed or otherwise reproduced it.