Meme

A meme was an idea which was transmitted between people. (AUDIO: Rule of the Eminence) It evolved as it was passed from person to person. (PROSE: The Book of the War)

The term meme was regarded by many as unnecessary since they argued that all ideas evolved as they passed from person to person, and therefore that "all ideas [were] by definition memes". (PROSE: The Book of the War)

Susan referred to the Library of Alexandria as being a "farm for memes" before remembering that memes were a concept from after Ian and Barbara's time. (AUDIO: The Library of Alexandria)

The human Sam Wentworth was once targeted by a dangerous meme in the postal system which took over his mind until it was inactivated by the First Doctor. As the Doctor noted, usually memes were random and mindless, but this one was an exception. (PROSE: INtRUsioNs)

Mictlan was made of "nothing but memes". (PROSE: The Book of the War)

An entity known as The Meme was a psychic worm built from the Cardiff Bay Intelligent Hotel and Spa by Jack Harkness. This particular repeated idea was based around the phrase "I Know What You've Done. I Know What You'll Do." As it was psychic, the meme could be weaponized to cause violence as it was shared across social media. (AUDIO:  Tagged)

Behind the scenes

 * While the word "meme" in modern vernacular usually refers to videos or images which are rapidly copied and spread on the internet, it is academically used to refer to cultural elements which are spread by a non-genetic means.
 * The word was modelled on the word "gene" and was coined by Richard Dawkins in his 1976 book.