A seed phrase (a.k.a. recovery phrase or mnemonic) is a sequence of 12 or 24 ordinary words generated when you create a self-custody wallet. It mathematically derives every private key in that wallet, so it is the single most sensitive piece of information you own in Web3. Never type it into a website, never photograph it, never store it in cloud notes.
Web3 & crypto basics
Seed phrase
The 12 or 24 words that recover a wallet; whoever has them controls the wallet.