Plain English Meaning
Easy-to-spot landmarks that don't have lights on them. During the day, you can use how clearly you can see these landmarks to judge how far visibility extends.
Full Definition
Conspicuous, unlit features on or near an airport or its approach path — such as towers, terrain, buildings, or trees — that are clearly visible during the day but not illuminated at night. In the context of visibility reporting and instrument procedures, they serve as visual reference markers used to estimate how far a pilot or observer can see horizontally in daylight conditions.
Context Anchor
Seen in FAA discussions of flight visibility, especially when describing how a pilot estimates forward visibility during the day.