Meme;
Noun:
A cultural unit (an idea or value or pattern of behavior) that is passed from one generation to another by non-genetic means (as by imitation)

Generally the word meme is used online to refer to those various quiz things that fill up 90% of the space on LiveJournal.

I think technically, memes refer to a sort of viral, compulsive running gag. IE, SPARTAAAAAA.
Longcat is Long, IT'S A TRAP, over NINE THOUSAND, THERAPIST, and so forth. But that's just my own technical definition based on the first or earliest time(s) I heard it used.

Let's see what the dictionary has to say!

meme (noun) coined by British biologist R. Dawkins in 1941. A unit of cultural information, as a concept, belief, or practice, that spreads from person to person in a way analogous to genes.

So at the least, those cultural tidbits that spread virally from one person to another.