Plain English Meaning
A runway that is thinner than the ones you usually land on. Because it looks smaller from the air, your eyes tell you that you are higher than you really are, and you may end up flying the approach too low.
Full Definition
A runway whose width is less than the width the pilot is accustomed to seeing on approach. On final, a narrower-than-usual runway projects a smaller image on the pilot's view, creating the visual illusion of being higher than the aircraft actually is, which can lead to a low approach and landing short.
Context Anchor
Seen during visual approaches and instrument training discussions about landing errors caused by optical illusions.