It's what happens when there's a mix of files from different firmwares, like how Frankenstein is a mix of different body parts.
Most common example is unintentional partial downgrades,
resulting in System firmware appearing to be 9.2, but unable to use any homebrew or entry hax.
I think a lesser known example might refer to people who purposely downgrade certain parts of their firmware,
such as reverting flashcart whitelist FW1.