The combining of light nuclei to form a heavier nucleus.
The production of all elements up to nickel (Ni) happens via the fusion process. Neutron bombardment of medium sized nuclei heavier than nickel produces heavier nuclei. These processes occur in stars, and are responsible for the presence of essentially all of the elements heavier than helium in the universe.
The condition in which the fusion power produced by the plasma equals the heating power put into the plasma. For a fusion power reactor to be economically useful, a burning plasma must produce much more fusion power than it consumes, thereby significantly exceeding the breakeven point.
The condition in which the fusion process has sufficient power to maintain the plasma temperature without external heat input. An ignited plasma burns on its own without external heat input in much the same way that an ordinary fire will burn on its own once a portion of a flammable fuel has been made sufficiently hot.