The story
From enormous + Lyman alpha + nebula. From being a nebula with Lyman-alpha emissions that is larger than galaxies.
Etymology from Wiktionary, CC BY-SA 4.0.
enormous Lyman-alpha nebula
Today it means: A large gas cloud (nebula) larger than galaxies, found in intergalactic space, which radiates with an emission spectrum containing 2.47 THz lines (Lyman-alpha lines). Full definition →