What is "fog line"

n. 1 The boundary of a body of air, contacting the earth's surface, that has fallen below the dew point and usually contains fog. 2 A line painted on a road (usually bright white) that marks the edge of the legally drivable portion. 3 (context computing English): The point in the stack descending into a portion of the program where there is no available source code, thus making debugging much more difficult. Most notably, the transition into proprietary code in a closed-source operating system.