The retina was a tissue lining the inner surface of the eye.
In 2153, Malcolm Reed was left with a detached retina following a fight with Major Hayes. (ENT: "Harbinger")
After Geordi La Forge's VISOR had filtered out an intense flash of light from the Cytherian probe in 2367, Doctor Beverly Crusher determined that his retina was undamaged. (TNG: "The Nth Degree")