Gray component replacement (GRC)
From The Glossary of Digital Photography
The replacement of neutral tones in an image with black to prepare it for four-color printing. Cyan, magenta, and yellow inks are replaced with black ink in areas where the tone is neutral. This is done to save ink usage and increase image detail. An image is processed during color separation with GRC. If done properly, the printing will use less ink and the image will look the same or better than if GRC were not used.
