Wiktionary
n. A special modifier key used to attach a specific diacritic to a letter previously typed.
Wikipedia
A dead key is a special kind of a modifier key on a mechanical typewriter, or computer keyboard, that is typically used to attach a specific diacritic to a base letter. The dead key does not generate a (complete) character by itself, but modifies the character generated by the key struck immediately after. Thus, a dedicated key is not needed for each possible combination of a diacritic and a letter, but rather only one dead key for each diacritic is needed, in addition to the normal base letter keys.
For example, if a keyboard has a dead key for the grave accent (`), the French character à can be generated by first pressing and then , whereas è can be generated by first pressing and then .
Usually, the diacritic in an isolated form can be generated with the dead key followed by space, so a plain grave accent can be typed by pressing and then . This legacy behavior is newly replaced with a more efficient one with respect to the use of combining diacritics following the Unicode Standard. The combining diacritic is then obtained with , while a spacing form of the diacritic is inserted with (typically + or +).
Usage examples of "dead key".
The sun was warm enough to sit outside, and after falling asleep several times in a lawn chair near the dead key lime tree in the backyard, Scarpetta was infected by the guilt she always felt when dealing with her mother: The grass was so thick it almost did not give beneath Scarpetta's weight when she walked across it, heading inside.