BIP-39 Passphrase (25th Word)

A BIP-39 passphrase, often called the 25th word, is an optional extra secret combined with your recovery phrase to derive a completely separate set of accounts. Adding it means the words alone are not enough to spend your funds, which protects against someone who finds or photographs the written backup. Any passphrase is valid, so each one opens a different wallet; some people keep a small decoy balance under the plain phrase. The trade-off is unforgiving: there is no recovery if you forget or mistype the passphrase, and no way to prove which one was correct. Back it up separately from the words, and test restoring before moving meaningful amounts.