What is a meme?

Merriam-Webster Dictionary provides two definitions for the term "meme":

1) an idea, behavior, style, or usage that spreads from person to person within a culture
2) an amusing or interesting item (such as a captioned picture or video) or genre of items that is spread widely online especially through social media

The term itself dates back to 1976, when the author and scientist Richard Dawkins first used it in his book The Selfish Gene.
In 2018, the word almost always refers to the second definition, an amusing item—most often a picture—that is shared mostly through social media. These memes carry ideas and behaviors that either tell us about our culture or become a part of our culture. Sometimes, the meme is a silly joke, like a pun. Other times, however, the meme makes a comment on politics or society in our time period. Memes will often use pictures of famous people or movie characters as part of the joke. The online magazine Thrillist ranked the top 50 memes of 2017 by their popularity and effectiveness.

Memes (discrete units of knowledge, gossip, jokes, and so on) are to culture what genes are to life. Just as biological evolution is driven by the survival of the fittest genes in the gene pool, cultural evolution may be driven by the most successful memes.

—Richard Dawkins
https://www.merriam-webster.com/dictionary/meme

https://www.thrillist.com/entertainment/nation/best-memes-2017