Octagon refers to actor and singer Jack Black starring in a segment from Sesame Street in which he teaches viewers what an octagon is. This segment has been used as a popular source for MADs and YTPMVs on both YouTube and Nico Nico Douga.
Jack Black Defines Octagon was uploaded to YouTube on July 18, 2008. Viewers quickly took notice of Black's very odd mannerisms and the fact that someone best known for the highly inappropriate Tenacious D and very raunchy works like Tropic Thunder is on a children's show and started using it to make YouTube Poops and memes.
Ask Jack Black, a National Geographic Kids video to promote 2008's Kung Fu Panda in which children ask Jack Black questions, is used alongside the octagon segment.
Octagon on YouTube and Nico Nico Douga
The first YTPMV using Octagon is "Octagons have ⑨ Sides" by MatrixMarioX, which is a remix of Beloved Tomboyish Girl from Touhou Project in 2010. It soon followed up on Nico Nico Douga after that video was uploaded there.
8 Sides of Nico Nico Douga, a MAD of Nanairo using the Octagon source, was uploaded to YouTube in 2012 to introduce Japanese MAD creators to the source. It was later followed by 8 Awesome Angles of YouTube!! in 2013 and The Glorious Octagon of Destiny in 2019.
The source is frequently referenced in the visuals for The Power of Terry.