Definition
Responsive to light. In the context of vision, describes specialized cells in the retina of the eye that react chemically when struck by light, generating the nerve signals the brain interprets as sight.
Plain English
Reacts to light. The cells in your eye that pick up light and let you see are photosensitive.
Context Anchor
Seen in discussions of night vision and the anatomy of the eye, especially when explaining how the eye responds to low light.
Derivation
From the Greek 'photo' meaning light, and the Latin 'sensitivus' meaning able to feel or respond. Together: able to respond to light. The same root appears in 'photograph' (light-writing) and 'photon' (a particle of light).
Why Pilots Care
The photosensitive cells in your eyes are what make night vision possible -- and what make it fragile. Understanding that these cells need time to adapt to darkness, and can be wiped out in seconds by a bright light, is the foundation of every night flying technique.
Intuition Check
Photosensitive does not mean “sensitive to photographs.” Here, photo means light, so photosensitive means responsive to light.
Example Sentence 1
The rods and cones in the retina are photosensitive cells that convert light into the signals your brain interprets as vision.
Example Sentence 2
A bright flashlight temporarily disables the photosensitive response, forcing the pilot to wait for recovery before continuing the flight.