Definition
A seed phrase (also called a recovery phrase or mnemonic) is a human-readable backup of a Bitcoin wallet's master secret, encoded as an ordered list of 12 or 24 common words. Standardized in BIP39, the words are drawn from a fixed list of 2,048 and map deterministically to the random entropy that seeds your wallet. From that single seed, the wallet derives every private key and address it will ever use, which is why the phrase — and only the phrase — is what you must protect and back up.
Why words instead of a raw key
The mnemonic encoding exists because humans transcribe words far more reliably than long hexadecimal strings, and BIP39 builds in a checksum so a mistyped or out-of-order word is usually detected on restore. Anyone who obtains your words can recreate your wallet and spend your coins, and anyone who loses them with no other backup loses the funds permanently. The phrase is therefore the single most sensitive object in self-custody.
Protecting and extending it
Best practice is to write the words on durable media (paper or stamped metal), never photograph or type them into an internet-connected device, and store backups in separate secure locations. A passphrase can be added as an optional 25th secret for a second factor, and Shamir's Secret Sharing (SLIP39) can split the backup into threshold shares so no single location is a point of failure.
The seed phrase is the root of self-custody; it is best generated and stored on a hardware wallet kept in cold storage, and it underlies every address along a BIP44 derivation path.
In Simple Terms
A seed phrase (also called a recovery phrase or mnemonic) is a human-readable backup of a Bitcoin wallet’s master secret, encoded as an ordered list…