The term 'Dead Letter' is used to refer to a letter or package that has been undeliverable due to incorrect or incomplete address information. The crossword solutions for this term are 'RUNE' (4 letters) and 'EDH' (3 letters). 'RUNE' is an ancient Germanic alphabet used for writing, divination, and magic. 'EDH' is an abbreviation for the Elder Futhark, which is the oldest form of the runic alphabet.