Photosensitivity: In medicine, increased sensitivity of the skin to light and other sources of UV.

Photosensitivity commonly causes reddening and blistering of the skin with exposure to sunlight and, in time, it heightens the risk of skin cancer. A number of diseases make for photosensitivity, as do scores of prescription and non-prescription drugs. (For details, see Sunburn and Sun-Sensitizing Drugs).

The eyes may also be oversensitive to light, a condition called photophobia, as occurs for example in measles.

In biology, photosensitivity need not be abnormal. It refers to the fact of being readily stimulated by light.