What are retinal ganglion cells?

Retinal ganglion cells represent the output neurons of the retina. They are responsible for integrating electrical signals that originate with the photoreceptors and, via their axons that comprise the optic nerve, transmit that information to higher visual centers of the brain. The retinal ganglion cells reside on the inner surface of the retina and their axons course across the inner surface to exit at the back of the eye through a region known as the optic nerve head.

This definition comes from:

Cell and Tissue Research
August 2013, Volume 353, Issue 2, pp 219-230
Autocrine and paracrine interactions and neuroprotection in glaucoma
by Arthur J. Weber