Dead key
A dead key is a modifier key on a computer keyboard and typewriter used to create a diacritic (accent mark) above a letter. For example, to create the letter "a" with the circumflex (^) diacritic using a German QWERTZ keyboard, press the ^ key (right of ß) and then "a" to get the "â" character. In programs like Microsoft Word using a US keyboard with no dead keys, the keyboard shortcut Ctrl+Shift+^+a would be used instead of a dead key.
For keyboards with a dead key, when you want to create only the symbol and not a letter with a diacritic, press spacebar after the dead key. For example, to create a circumflex (^) by itself, press the ^ key (right of ß) and then the spacebar.
Unlike other modifier keys (e.g., Shift), a dead key does not need to be held down. Instead, it modifies the next key you press. Nothing appears to happen when the dead key is pressed, but the keyboard is remapped, so the next letter pressed is a diacritic letter.
Why is it called a dead key?
When the acute accent was introduced on the mechanical typewriter, the key did not move the paper like all other keys. This lack of movement made the key appear as if it was dead (a dead key).
In addition to being called a "dead key," it was also called a "non-spacing key."