A seed phrase is a collection of words that can be used to access your cryptocurrency wallet.
You’ll often hear the terms mnemonic seed or mnemonic phrase used to describe the same idea. Some believe that these are poor descriptors, however, as they imply that the phrase needs to be memorized.
Introduced in Bitcoin with BIP39, seed phrases provide users with an easier way of backing up their wallets. If you’ve used a wallet previously, you’ve likely been prompted to write down 12-24 random words like the following:
forget wing follow flip swallow achieve correct view dinner witness hybrid proud
If ever you lose access to your wallet, these can be loaded into any BIP32-compatible wallet to recover your funds. There are 2048 words on the BIP39 wordlist, meaning that a string of 12 words would have 128 bits of security. This means that an attacker would need to perform 2128 operations to guess a 12-word seed. It may look like a small number, but it certainly isn’t. It’s practically infeasible for even the most well-resourced attackers to crack this sequence (for now). The longer the seed, the more secure it is.
Seed phrases – as opposed to private keys – leave less room for human error when it comes to backups, as they're easier to record and transmit. Better still, they’re capable of generating not just one, but a colossal amount of keys from a master one. This allows users to avoid address reuse as, with a single seed, they can create a seemingly infinite number of receiving addresses.
Seed phrases are not cryptocurrency-specific. They can be used across hundreds, so that one seed can actually be used to recover an entire portfolio of coins and tokens. The majority of leading wallets allow the user to recover their funds with a seed phrase, provided they have correctly recorded it.