A meme is "an idea, behavior, or style that spreads from person to person within a culture."

 A meme acts as a unit for carrying cultural ideas, symbols, or practices that can be transmitted from one mind to another through writing, language, gestures, rituals, or other imitable phenomena.

A macro image style  internet meme. image: R.Conan-Davies/imgur

The term was coined by Richard Dawkins in his book 'The Selfish Gene"published in 1976. A meme is considered a unit of human cultural evolution analogous to genes.

It is now most commonly associated with a style of internet image (macro image) and overlaid text.