In land surveying, a witness point is a monument that is located a specific distance and bearing from a second point that itself cannot be monumented for some reason. For example, if a true property corner happened to fall, say, in the bottom of a pond, the surveyor would set a witness point on dry land that served as a reference point used to locate the true property corner.
This has already been answered, but I just wanted to add that it is similar to benchmarking, in which surveyors would use some sort of marker to, say, mark an elevation