1.
vengeful spirit, wrathful ghost
The spirit of a deceased person who harbors deep resentment and returns to the world of the living to cause harm, disaster, or misfortune. A fundamental concept in Japanese folk religion and a recurring motif in Japanese literature and horror.
Cultural note
Belief in 怨霊 has been a powerful force in Japanese history and culture. When powerful or unjustly treated people died with grudges, their spirits were believed to cause plagues, natural disasters, and political upheaval. To appease them, they were often enshrined as deities (御霊信仰, goryo shinko). Famous historical 怨霊 include 菅原道真, 平将門, and 崇徳天皇.
Do not confuse with the homophone 音量 (volume, loudness), which uses different kanji.