A keyline, in graphic design, is a boundary line that separates color and monochromatic areas or differently colored areas of printing on a given page or other printed piece. The line itself, usually consisting of a black (or other dark colored) border, provides an area in which lighter colors can be printed with slight variation in registration. ...
outline showing shape and position of an illustration
Alternate term for Mechanical.
An outline drawn or set on artwork showing the size and position of an illustration or halftone. 
A line drawn on artwork which indicates an area for tint-laying, positioning of half-tones, etc. where this must be done at a later stage.
the line that runs around the outside of a piece.
Can be any one of many partial or complete descriptions of past buying history codes to include name-and-address information and current status.
Lines that are drawn on artwork that indicate the exact placement, shape and size of elements including halftones, illustrations etc.
the outline around a whole graffiti piece