noun
identifying mark; comradeship mark
A mark used to identify members of a group or to show alliance; often a secret sign among comrades.
彼らは合印として赤いリボンを身につけていた。
They wore red ribbons as an identifying mark.
裁縫で、布に合印を付けてから縫い合わせる。
In sewing, you mark alignment marks on the fabric before sewing them together.
noun
A seal or mark used for verification, often in official or commercial contexts; also refers to a tally used for counting or matching. The reading あいいん is a variant for this sense.
See also: 合い印 (あいいん)
帳簿と現金が合っていることを示す合印を押す。
They stamp a verification seal to show that the books match the cash.
Compound of 合い (ai, 'mutual, together') + 印 (shirushi, 'mark, sign'). The exact historical derivation is uncertain, but the word has been used since at least the Edo period for marks of identification and alignment.