Plain English Meaning
What you see out the window can sometimes trick you into thinking the airplane is higher, lower, more level, or moving differently than it really is. These tricks of the eye are called visual illusions.
Full Definition
Misleading impressions of altitude, attitude, drift, speed, or position that arise when a pilot interprets the outside scene incorrectly. They occur because the visual cues a pilot uses in flight — the horizon, terrain features, ground patterns, runway shape — can be distorted by perspective, lighting, weather, terrain slope, or unfamiliar shapes, causing the pilot to misjudge what the airplane is actually doing.