A meme is a concept of any sorts (image, video, song, etc.) that spreads from person to person (through the Internet, popular culture, etc.) within a culture, to the point that it becomes part of that culture, acting in a way as the cultural analogue to genes (at least, in the more traditional sense of memes, not the current usage).
There are a plethora of memes; for a full list of those predominantly featured, see tag group:Meme.
This tag is automatically added along with any tag with a name ending with *_(meme). When creating a new meme tag, make sure to follow this format.