Seed phrase (recovery phrase)

Modern wallets derive all their keys from 1 random seed. The BIP39 standard turns that seed into 12 or 24 words from a fixed list of 2,048, so that a human can write it down and type it back. Restore the words into any compatible wallet and the same keys, addresses and coins reappear.

This makes the phrase the single point of failure of self-custody. It must be written offline, never photographed, never stored in a cloud, and protected from fire, water, theft and heirs who do not know what it is. An optional passphrase, sometimes called the 25th word, derives a different wallet from the same words. It defeats a thief who finds the paper, and locks you out for good if you forget it.