Definition of "unch"

unch¹

noun
plural unches
1. (crossword puzzles) An unchecked square: one that is part of only one entry (i.e., across or down, but not both).
  Quotations
  The Sympathy Crossword Grid Editor supports:...o flagging of lights outside a configurable unch range
  1994 September 24, Ross Beresford <ross@bryson.demon.co.uk>, “symp10.zip - Crossword Grid Editor for MS Windows 3.1”, in comp.archives.msdos.announce (Usenet)
  From 1933 onwards the grids were standard grids. Then, for some reason, there was a blip in the late 1940s, a couple of the grids from that era had double unches and double checking.
  2005 November 17, Colin Blackburn <colin.blackburn@durham.ac.uk>, “Re: 75 Years of the Times Crossword - quick review”, in rec.puzzles.crosswords (Usenet)
  The bars and multitude of black squares are common in British puzzles because they allow unchecked letters (called unches)—or letters that don't need to cross with another answer.... There are rules regarding the unches, and traditionally every other letter in a solution should be checked with another letter.
  2006, Nikki Katz, Zen and the Art of Crossword Puzzles, Adams Media, pages 66–67

unch²

adjective
not comparable
1. (stock markets) Abbreviation of unchanged (“of a stock price: having closed at the same price at which it opened”).