True weightlessness: Only at infinite distance from all masses (theoretical).

Apparent weightlessness: When the normal force (or support force) is zero. This happens:

1. In a freely falling elevator: a = g, so N = m(g-a) = 0
2. In an orbiting satellite: both the satellite and astronaut are in free fall around Earth
3. At the centre of Earth: g = 0 (all mass pulls equally in all directions)

Not weightless: An astronaut in orbit has weight (mg is not zero at that height). They experience apparent weightlessness because the satellite and astronaut have the same acceleration (free fall).