A seed phrase (also called a recovery phrase or mnemonic) is a list of 12 or 24 ordinary words generated by your crypto wallet when you first set it up. Behind the scenes, these words encode your private key in a form that is human-readable and easier to back up than a raw key.
Anyone who has your seed phrase has complete control of your wallet and everything in it. There is no two-factor authentication, no fraud department, no reversal. A seed phrase is the master key and must be treated with more care than a bank card PIN.
Write it down on paper and store it in a safe, offline place — multiple copies in different locations if the stakes are high. Never type it into a website or screenshot it. Legitimate software and hardware wallets will never ask for your seed phrase after setup.
Worked example
Your wallet might generate a phrase like "apple branch clever diamond" (truncated) — those words, in that order, recreate your entire wallet on any compatible device.
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This definition is general education, not investment advice. Markets — especially crypto — are volatile and you can lose money. Please read our disclaimer and see our methodology.