What is a BIP-39 passphrase?
An extra word or sentence added to recovery words, which creates a completely different wallet. Losing it means the words alone cannot help.
Not to be confused with: the recovery words themselvesRecovery words (seed phrase)A list of 12, 15, 18, 21 or 24 words that can restore wallet keys. Anyone who has them can take your bitcoin..
BIP-39 lets you add a passphrase of your own choosing on top of your recovery wordsRecovery words (seed phrase)A list of 12, 15, 18, 21 or 24 words that can restore wallet keys. Anyone who has them can take your bitcoin.. The same words with a different passphrase, or none, derive different keys: a separate wallet with its own addresses and balance.
People use one as a second secret, or to keep a decoy wallet on the words alone. A wrong passphrase does not fail: it opens a different, usually empty, wallet. Keep it written down separately from the words.
References
- Bitcoin Improvement Proposals, “BIP-39: Mnemonic code for generating deterministic keys”.