Niflheim was the opposite world with Muspelheim the Fire Land. Norse Niflheim was the land of ice and mist.
Niflheim was one of Nine Worlds in Norse mythology. The Old Norse name of it was Niflheimr (NIF-el-hame). The most famous meaning of Niflheim was the Kingdom of Mist. The Norse Niflheim was the land of primeval element, ice. This world was full of darkness, mist, fog, and ice. Because of this, we can call Niflheim the opposite world with Muspelheim. The similarity between the two was that they both had a crucial part in forming the first giant Ymir (See Creation of The World).
One of three holy wells watering Yggdrasil was located in Niflheim. The name of it was Hvergelmir. From the well Hvergelmir came other eleven rivers. Hvergelmir was the origin of all living and all the living also came back to it.
The name “Niflheim” was assumingly the late work from Prose Edda by Snorri Sturluson. But the exact connotation of it is somewhat vague and still remains to be seen.