A diacritic is a mark or sign added to a letter to indicate a different pronunciation, stress, or meaning. The three diacritics mentioned in the crossword solutions are the tilde (5 letters), cedilla (7 letters), and macron (6 letters). The tilde is a curved line placed over a letter, usually an 'n' or 'a', to indicate a nasal sound. The cedilla is a small hook placed under the letter 'c' to indicate a soft 's' sound. The macron is a straight line placed over a letter, usually an 'a' or 'e', to indicate a long vowel sound.