Definitions for "Main sequence star"
The central band of stars on the Hertzsprung-Russell Diagram. These stars' energy comes from nuclear fusion, as they convert Hydrogen to Helium. Most stars are Main Sequence Stars. For these stars, the hotter they are the brighter. The sun is a typical Main Sequence star.
A star whose brightness and temperature place it on the Main Sequence, in the Hertzsprung-Russell Diagram. As it happens, a star whose energy source is the fusion of hydrogen to helium, either through the proton-proton cycle (for lower Main Sequence stars), or the carbon cycle (for upper Main Sequence stars).
a star that is in its normal state, such as the sun