Unch, short for unchecked letter, refers to a square in a crossword puzzle which contains a letter from an Across
answer but not a Down
answer (or vice versa). This means the solver cannot check the veracity of the letter by solving the Across or Down clue. An unchecked letter is common in British-style crosswords
but is not allowed in standard North American puzzles.
Also Known As: Unchecked letter, unchecked square