A version of 5G that relies on a corresponding 4G network to function. The 4G network is the primary network, handling voice, text messaging, and setting up data sessions that will happen over 5G.
NSA is a kind of bridge technology that allows the deployment of 5G data service before the other aspects of 5G technology are fully ready. The first wave of 5G networks in the US all use the NSA version of 5G. They will later be upgraded to the SA (Stand-Alone) version of 5G, which does not rely on 4G networks.
See: 5G NR