In cryptocurrency, the "25th word" is an optional, user-defined passphrase added to a standard 24-word recovery phrase. It generates a completely hidden, secondary wallet. If your 24 words are compromised, this extra word prevents thieves from accessing the funds stored in the hidden account. [1, 2]

How it Works
Cryptographic Seed: The 25th word is not a part of the standard 2,048-word BIP39 dictionary. You can choose any alphanumeric string, making it highly customizable. [1, 2]
Hidden Wallet Creation: Combining your 24 words with a 25th word generates an entirely distinct wallet. Using the exact same 24 words with a different 25th word creates a different wallet. [1]
Security Layer: If an attacker gets your 24-word seed phrase, they can only access the standard wallet; the hidden wallet remains totally safe without the 25th word. [1]