"Fiend" is a colloquial term used by those of The Inner Planes. The term is used in reference to Celestials which invade The Inner Planes and cause strife for the plane's inhabitants.

Relations of other Races
Everyone prefers to give Fiends a wide berth. Only the most desperate will seek to make a contract with a devil or force a demon into service.

Lore
Legends say that Fiends were the first creatures to take interest in The Material Plane. Devils sought to incorporate the plane into their rigid hierarchy, making use of its resources. Demons sought rampant destruction, deeply offended by this stain on The Universe.