Is 'lateral inhibition' just a term for the biological basis of the functioning of the on-center (or off-center) ganglion cells?
Or do these terms describe separate processes?
Retinal center-surround receptive fields are an example of lateral inhibition. It occurs elsewhere in the nervous system too.
Center-surround receptive fields are indeed an example of lateral inhibition, where the ON field suppress the OFF field through lateral inhibition. The center-surround connectivity in the retina (Fig. 1) is indeed the most well-known example of this kind of circuitry. However, lateral inhibition occurs in other sensory systems too, for example auditory and olfactory neurons (Bakshi & Ghosh, 2017). For more information on the retinal circuitry underlying center/surround inhibition see this answer.
Fig. 1. Retinal circuitry underlying On-OFF center-surround conncetivity in ganglion cells. source: New York University