fog line

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The boundary of a body of air, contacting the earths surface, that has fallen below the dew point and usually contains fog.
• noun A line painted on a road (usually bright white) that marks the edge of the legally drivable portion.
• noun computing : 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.