Dawkins used the term to refer to any cultural entity that an observer might consider a replicator. He hypothesised that one could view many cultural entities as replicators, and pointed to melodies, fashions and learned skills as examples. Memes generally replicate through exposure to humans, who have evolved as efficient copiers of information and behaviour. Because humans do not always copy memes perfectly, and because they may refine, combine or otherwise modify them with other memes to create new memes, they can change over time. Dawkins likened the process by which memes survive and change through the evolution of culture to the natural selection of genes in biological evolution.
Dawkins defined the meme as a unit of cultural transmission, or a unit of imitation and replication, but later definitions would vary. Memes, analogously to genes, vary in their aptitude to replicate; memes that are good at getting themselves copied tend to spread and remain, whereas the less good ones have a higher probability of being ignored and forgotten. Thus “better” memes are selected. The lack of a consistent, rigorous, and precise understanding of what typically makes up one unit of cultural transmission remains a problem in debates about memetics. In contrast, the concept of genetics gained concrete evidence with the discovery of the biological functions of DNA. Meme transmission does not necessarily require a physical medium, unlike genetic transmission.