A crossword puzzle's (usually square) grid has white squares and black squares. I define a crossword grid's "density" to be the ratio of the number of white squares to the total number of squares. Clearly, the larger the density the harder the crossword will be to construct, with a ratio of one being a grid with no black squares at all.
For example, a recent New York Times crossword is a 15-by-15 square with 42 black squares and 225-42=183 white squares for a "density" of 183/225, approximately 0.81.
Is there a term of art for what I am calling crossword puzzle density? If so, can someone provide a source for that term?