2channel (2ちゃんねる, ni channeru; 2ch for short) - also known as 5channel (5ちゃんねる, go channeru; 5ch for short) since 2014 split.
A Japanese internet forum, often thought to be the largest in the world in its prime. It has over 600 boards covering a wide range of topics, as their slogan ('From "hacking" to "side dishes for tonight's dinner"') would suggest.
2ch/5ch plays a large part in Japanese internet culture and has created a culture and language in and of itself. ASCII and Shift-JIS art is popular there and many meme/neta and characters have been created based on it. In addition, several internet celebrities have been born there, as have many collaborative projects and pranks (ie: Densha Otoko, Buront, Kasane Teto and the Vippaloids). However, not unlike 4chan of the west, 2ch/5ch is infamous for its near anarchic, non-existent moderation, which results into the site being a hotbed for political extremists and illegal activities.
In 2014, after the dispute about the domain ownership, the original 2ch.net domain was seized by a group of American shareholders led by Jim Watkins and renamed to 5ch.net, whereas the original founder of 2ch, Nishimura Hiroyuki (who went on to purchase 4chan domain and renamed it "4channel") later opened a "successor" site based in Singapore with the registered domain of 2ch.sg. Both sites are still disputing over their claim of being the "legitimate" successor of 2ch.net to this day.