In scanning, especially the scanning of line art, a setting used to determine the level of gray at which a particular pixel is interpreted as black or white. In 1-Bit color, pixels can only be one of those two values. Setting the threshold enhances the ability of the device to capture shades of gray.
In graphics and image manipulation programs, threshold refers to a setting controlling how many pixels in the image are affected by a particular operation, in particular various forms of edge enhancement such as unsharp masking.