Obsidian pillars were tall towers made of obsidian in The End, which had floating crystals atop them. Some of these towers were stood upon by the Fourth, Sixth, and War Doctors during a battle against a dragon. (WC: Doctor Who Comes to MINECRAFT!)
Behind the scenes
Other names for the 'obsidian pillars' in Minecraft included "End spikes", "obsidian spikes", "obsidian towers", and "End pillars".