noun
backdrop; backcloth
Refers to the large painted cloth or curtain hung at the back of a theater stage or similar setting.
舞台の背景幕が静かに上がった。
The stage backdrop rose quietly.
背景 is a general term for background (visual, social, or contextual), while 背景幕 specifically means a physical backdrop cloth or curtain used in theater.
Compound of 背景 (background) and 幕 (curtain). The exact historical derivation is uncertain, but the word transparently describes a background curtain.