Viral Internet idea (4)
I believe the answer is:
meme
'viral internet idea' 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 "Cultural belief that assumes a life of its own" , "online phenomenon" , "Image etc. that spreads rapidly on the Internet" , "Concept that spreads rapidly on the Internet" , "Behavioural trait" .)