Plain English Meaning
The cells in your eye that actually 'see' light. They pick up light coming in and turn it into signals your brain reads as an image.
Full Definition
Specialized cells in the retina at the back of the eye that respond to light and convert it into nerve signals sent to the brain. In the human eye, these are the rods and cones — rods handle low-light and peripheral vision, while cones handle color and fine detail in good light.
Context Anchor
Seen in night vision and eye anatomy discussions, especially when explaining why vision changes in darkness.
Derivation
From Greek 'photo' meaning light, and 'sensitive' meaning able to detect or react to something. Literally: cells that detect light. The word itself tells you what they do.