Retinal detachment: A separation of the retina from its connection at the back of the eye. The separation usually results from a tear (that is, a rent or rip, not a tear drop) in the retina, which often occurs when the vitreous gel pulls loose or separates from its attachment to the retina. Once the retina has torn, the vitreous liquid can pass through the tear and accumulate behind the retina. The build-up of fluid behind the retina is what separates (detaches) the retina from the back of the eye. Vision loss occurs if the detached retina is not repaired.