scented insert; perfumed paper for letters
A small piece of paper infused with fragrance, traditionally enclosed with a letter to impart a pleasant scent.
手紙に文香を添えると、受け取った人が開けたときにほのかな香りが広がります。
When you enclose a scented insert with a letter, a subtle fragrance spreads when the recipient opens it.
Compound of 文 (ふみ, 'letter') and 香 (こう, 'fragrance'). The exact historical origin is uncertain, but the term refers to a traditional scented paper used in correspondence.