noun
A stiff paper or board used to support or frame a photo, painting, or document. Often refers to the decorative border in framing.
写真を台紙に貼って額に入れた。
I mounted the photo on backing paper and put it in a frame.
この台紙は厚みがあってしっかりしている。
This mount is thick and sturdy.
Compound of 台 (stand, base) and 紙 (paper). The exact historical derivation is uncertain, but the word literally means 'base paper' and has been used in framing and mounting contexts.