Seed phrase is the modern alias for the BIP-39 mnemonic phrase. Some wallets prefer "seed phrase" (Ledger, Trezor), others prefer "recovery phrase" (Coinbase Wallet), and a few stick with "mnemonic" (Sparrow, Specter). All three refer to the same object: the twelve or twenty-four words that encode the entropy from which your wallet derives every key.
Why three names exist
"Mnemonic" came from the original 2013 BIP-39 specification. "Seed phrase" emerged in wallet UX around 2017 because casual holders found "mnemonic" technical and unfriendly. "Recovery phrase" came later — Coinbase Wallet led the rename — to emphasize the actual use case rather than the underlying math.
For a US-resident reader, the practical advice is: when an exchange or wallet asks for any of the three, treat all three as identical. Treat a "recovery phrase" exactly the way you treat a "private key" backup. There is no security difference; there is only a marketing difference.
The standard length
Twelve words encode 128 bits of entropy plus four checksum bits. Twenty-four words encode 256 bits plus eight checksum bits. Both lengths are secure against brute-force attack for the foreseeable future. The argument for twenty-four words is forward security against advances in classical computing power; the argument for twelve is operational simplicity. Trezor, Ledger, and Coldcard default to twenty-four; MetaMask defaults to twelve.
What the words are not
The words are not a password you can set. They come from a fixed 2,048-word English wordlist standardized by BIP-39. The wallet does not let you "pick easier words." The words are also not English in the sense of grammar — the order matters but the meaning does not. "Apple zebra wrist..." is just as valid as "amount donate journey..."
The single rule
The seed phrase enters a device exactly three times in its useful life: when the wallet generates it (write it down), when you restore it to a new device (verify the words came back), and when you rehearse the restore (every six to twelve months, on a clean device, with a small test balance). Anything else — typed into a website, photographed, dictated to "support" — voids the security model.
Further reading: Private keys and seed phrases, Five ways to store a seed phrase, BIP-39.