Crypto glossary
Passphrase (25th word)
A passphrase, or 25th word, is an optional secret added to a seed phrase that creates a completely separate wallet; without it, the seed alone opens nothing.
BIP-39 allows users to add any text as a passphrase when turning a seed phrase into a wallet. Each different passphrase, even one differing only in capitalisation or a space, produces an entirely different set of keys and addresses. The seed phrase without the passphrase opens a separate, usually empty or decoy wallet.
The feature protects against physical theft of the seed backup: a burglar who finds the 24 words still cannot reach the funds without the passphrase, which can be memorised or stored elsewhere. It also offers plausible deniability, since a user under duress can reveal a passphrase that unlocks a small decoy balance. Most hardware wallets support it.
The risk is losing it. The passphrase is not stored in the seed, cannot be recovered and has no checksum, so a single typo opens a different, empty wallet instead of returning an error. Users have locked themselves out permanently. Anyone using one should back it up as carefully as the seed, in a different place, and test recovery before depositing funds.
Example
A user stores their 24 words in a safe and memorises a passphrase; when a burglar steals the paper and imports it, they find only an empty wallet, while the real savings sit behind the passphrase.