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1.
An internet celebrity is an individual
whose fame is primarily derived from their Internet presence. Many of the top web
celebs have a proven following that rivals popular television programs.
Specifically, the iStardom Top 10 has a fan base exceeding 8 million people.

Web celebrities are an entirely new breed of stars and ones that have rising
influence," said Daisy Whitney, TelevisionWeek's new media reporter and host of
the New Media Minute. Brand marketers, production companies, and
traditional media should pay attention to who's hot online and find creative
ways to partner with them.
An in internet celebrity is someone whose primary source of fame originates from the Internet." This is in contrast to news sources, movies, TV, books, etc.
by clockiop February 14, 2009
 
2.
someone who would never be famous in real life but, due to the magic of the internet, has become a recognizable name in chat rooms and forums.
tom from myspace, drew curtis(fark), star wars kid, icy hot stuntaz, pixieland guy, etc.
by ray March 25, 2005
 
3.
Someone whose credibility ends where real life begins.
"My YouTube video has the square root of pi trillion hits. That makes me an Internet celebrity!"

"Shut up and give me my taco, you loser fuckbrick."
by Gerbil Warrior April 17, 2008
 
4.
Someone who is known (either world-widely or nationally) through the internet, though mainly from Youtube. Many get hated especially if they breakthrough and be part of Hollywood business (Justin Bieber, Rebecca Black), but a lot are admired.

Most internet celebrities are comedians, singers, bloggers and, nowadays, pranksters. The singers part is where some haters grow, but the comedians get a lot of praise.

They usually start out good and funny, but after a few videos, they start to do anything they want and lose ideas/creativity. But people still like them. I admit, I'm one.
Janoskians, Chris Crocker (one of the first, most influential), Shane Dawson, Smosh, Nigahiga, MrTeddy, Kevjumba, Ray William Johnson, Perez Hilton, Jenna Marbles, Community Channel (Natalie Tran)...are examples of an internet celebrity
by kilamnyaz June 06, 2012