Ghosting
Also called keyboard ghosting
Ghosting is when pressing a certain combination of keys makes a keyboard report an extra key you never pressed. It comes from the way cheaper keyboards wire their keys in a shared grid.
True ghosting is rare on modern keyboards because they block the phantom key instead. What most people call ghosting today is key blocking.
In practice
Most keyboards wire their keys in a grid of rows and columns. Press three keys at the corners of a rectangle on that grid and the circuit can make the fourth corner look pressed too. That phantom press is ghosting. Few modern keyboards let it through. Instead they block the extra key, which is why people now use ghosting to mean any key that drops out in a combination.
How to check
Hold combinations of three or four keys in the keyboard test and look for a key that lights up without being pressed. That is true ghosting and it is rare. A key you are pressing that refuses to light while others are held is key blocking. Both mean the keyboard lacks per key diodes and the fix is a keyboard with better rollover.