Dead key is used for inputting characters with diacritic marks, such as: éàêñü.
Press (and release) the dead key first, then type a letter.
For example of keyboard with dead key, see:
Many European language layout have both Alt Graph Key and dead keys. See: International Keyboard Layouts .
On the Mac, ⌥ option+e is a software dead key. It adds the acute mark to any letter typed after.