Noun
A contour line connecting all points in valley at the same elevation as the keypoint, used in planning efficient water usage.
(graphic design) A boundary line that separates color and monochromatic areas or differently colored areas of print.
Source: en.wiktionary.orgCommon reasons for using black ink include: citation * In traditional preparation of color separations, a red keyline on the black line art marked the outline of solid or tint color areas. Source: Internet
Because usually the black plate contained the keyline, the K in CMYK represents the keyline or black plate, also sometimes called the key plate. Source: Internet
In some cases a black keyline was used when it served as both a color indicator and an outline to be printed in black. Source: Internet