Definition
Visibility conditions that are barely adequate for safe visual flight, where the pilot can see outside the aircraft but the visual references needed to maintain orientation, attitude, and terrain clearance are reduced, intermittent, or unreliable.
Plain English
You can still see out the window, but only just. There's enough haze, mist, low light, or obscuration that you can't fully trust what you're seeing to keep the airplane upright and on course.
Context Anchor
Seen in discussions of spatial disorientation, especially when a pilot is flying with weak outside visual references rather than a clear horizon.
Derivation
Marginal comes from the Latin margo, meaning edge or border. Marginal visibility is visibility right at the edge — just barely enough to fly visually, with little room before it becomes unsafe.
Why Pilots Care
Recognizing marginal visibility helps a pilot decide whether to continue visually or switch to instruments before spatial disorientation sets in.
Grounding Statement
Picture flying into haze where the ground is faint and the horizon almost blends into the sky; you are not blind, but your eyes no longer give a solid picture.
Intuition Check
Marginal visibility does not mean visibility is acceptable with only a small problem. It means the view is close to not being enough, and outside cues may be unreliable.
Example Sentence 1
With marginal visibility along the route, the pilot decided to file IFR rather than continue VFR into worsening haze.
Example Sentence 2
With marginal visibility, the horizon became indistinct, so the pilot cross-checked the attitude indicator more frequently.