1. A technology that allows mobile phones or other devices to connect directly to mobile networks that are not land-based. Generally this means space-based networks (satellites orbiting the earth), but it can also mean base stations at very high altitudes, such as attached to balloons or planes. This technology allows phones (and other devices) to have service where land-based cellular networks do not provide coverage.
These technologies are usually limited in capability, offering only text messaging (one-way or two-way) or very limited data bandwidth.
Note that NTN usually refers to a technology that must be specifically designed into a phone; not all phones support NTN. It also may use a special radio frequency not normally used for cellular service. There are other approaches that offer similar service but work with essentially all phones, using standard cellular frequency bands.
2. The 3GPP standard for NTN (as described above) with 5G technology. This standard, called NR-NTN, encompasses satellite networks operating at LEO, MEO, or GEO orbits.