The convection zone of a star (e.g., the Sun) is that interior region where energy is transferred outward through convection, energy moved outward through the movement of hot material. This happens when it expands due to high temperature, and its density falls below that above it, an instability, and it is pushed away from the center of gravity through buoyancy. This occurs when opacity is sufficiently high that radiative transfer is too inefficient to move the energy and relieve the heat within the lower layers. In general among main sequence stars, the smallest, such as red dwarves have convection zones through much of the star, mid-sized stars, such as the Sun, have a convection zone in the outer portion, and massive stars such as O-type star have no convection zone.
Similarly, the radiation zone is the interior volume where radiative transfer is dominant and the term radiative-convective boundary (RCB) indicates the border between the zones.
The terminology is used for planets as well, particularly gas giants such as Jupiter and Saturn. I've seen the term RCB used more often for planets and the term convection zone more often for the Sun.