It's a called a 25th word intentionally
It's intentional, but it's misleading. Your passphrase does not just have to be a word, and indeed, shouldn't just be a word since a single word is easily brute forced and not secure. Passphrase or seed extension is better since it is more clear that you can use a whole phrase or numbers and symbols as well, which are necessary if you want it to be secure. It's also misleading to call it a 25th word since you can use it with seed phrases of any length.
BIP39 refers to it as a passphrase, although BIP39 also refers to seed phrases as mnemonic codes which I strongly disagree with since it implies they should be memorized.
It's not the same as the "passphrase" used to refer to a password you use to encrypt your wallet file.
I would refer to that as a decryption key to avoid confusion.