Loli or Lolikon is a type of Japanese animated pornography (all types, including manga, film and general images). It refers to the age of the main interest in the art, which is generally pre-teen. It has been expanded to refer to any girl under the age of about 15, real life or other.
Is this loli?
Is this loli? is a meme that originated from a reply to people posting jailbait back in '05/'06, when it was essentially non-existent on 4chan.