Passphrase
A passphrase is an optional secret mixed in when a seed phrase is converted into a wallet seed. Sometimes called the "25th word", though it is neither a word from the BIP39 list nor appended to the phrase.
The consequence people underestimate: a different passphrase produces a completely different wallet, not a locked version of the same one. That makes it powerful, since finding your written words is then not enough, and dangerous, since forgetting it loses the funds as surely as losing the phrase. Store it separately from the phrase itself - see What is BIP39?
In everyday password usage, "passphrase" also describes a password built from several random words, which is longer and easier to remember than a short complex string - see What is a Strong Password?