From WoWWiki

Idrassil is the name of a concept art in The Art of World of Warcraft.^{AoWoW 120} It depicts a World Tree growing out of an island. There are currently only two known world trees on Azeroth, and they are Nordrassil and Teldrassil respectively. Given only one of them grows out of an island, it is likely that Idrassil is an earlier name of Teldrassil.