noun
signature specimen; signature sample
A sample of a person's signature kept for verification purposes, such as in banking or official documents.
銀行で口座を開くとき、署名鑑を登録する必要がある。
When opening a bank account, you need to register a signature specimen.
サイン is a general term for a signature, often used casually, whereas 署名鑑 specifically refers to a registered signature specimen for verification.
Compound of 署名 (signature) and 鑑 (model, specimen). The exact historical derivation is uncertain, but the term is used in formal contexts for signature verification.