What is steam in thermodynamics?

Saturated steam is gas-phase water in equilibrium with liquid water at the same temperature and pressure. If the temperature is above the saturation point and/or the pressure is below the saturation pressure, the steam is "superheated steam". In either case it is a gas.

There are some conventions where saturated steam along with some fraction of liquid water in equilibrium with it is still referred to as steam even if not all of it is gas, as long as at least some of it is gas.