witness mark n. generally, an intentional, accidental, or naturally occurring spot, line, groove, or other contrasting area that serves as an indicator of certain facts; in geography and surveying, a blaze, cut, hole, paint splash, or message written on a post, tree, rock or other guide to indicate a boundary, feature, or significant point on land, especially on a witness post; in construction and manufacturing, a line, groove, score, notch, cut, or written indicator made on the surface of material to impart information, such as where to cut or join; in forensic investigation, a surface groove, smear, stain, abrasion or other feature that can serve as evidence. (source: Double-Tongued Dictionary)
Further reading
Spredge is a portmanteau word, a blend of sprayed and edge, and refers to decorative spraying of the edges of book pages, a trend in book design. These are historically similar to fore-edge paintings. This is part of a complete episode.
Related
Responding to our discussion about thin places, those spots where the boundary between this world and other realities seems narrow or permeable, a listener in Kirkland, Washington, sent us some eloquent thoughts about her own experiences of that...
In process engineering, a witness mark can be the result of a particular process, the presence of which will prove that the process has occurred. E.g. A mark left on a test point from a test probe. However, in some circumstances, such marks are considered to be damaging and so efforts are made to avoid, prevent or eradicate such marks.