A traditional Japanese emblem used to identify a family, often seen on formal kimono, gravestones, and historical artifacts.
彼の着物には家紋が入っていた。
His kimono had a family crest on it.
お墓に家紋が刻まれている。
The family crest is engraved on the gravestone.
家紋 specifically means a family's own crest, while 紋 can refer to any crest, pattern, or even a card suit.
Compound of 家 (ie, 'family') and 紋 (mon, 'crest' or 'pattern'). The term has been used since the Heian period to denote hereditary family emblems.