Viral phenomenon (4)
I believe the answer is:
meme
'viral phenomenon' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for meme that I've seen before include "online phenomenon" , "Question that spreads on the internet" , "Transmitted cultural idea" , "Image etc. that spreads rapidly on the Internet" , "Often trite image" .)