The Collaborative International Dictionary of English v.0.48:

main sequence \main" se`quence\, n. (Astronomy)
   That region on a two-dimensional graph of luminosity versus
   temperature for stars (the Herzsprung-Russel diagram), which
   runs from high temperature and high luminosity to low
   temperature and low luminosity, in which most of the stars
   (plotted as points on the diagram) are found. A normal star
   such as the earth's sun will spend most of its time over
   billions of years within this region of temperature and
   luminosity, as it progressively converts more of its original
   hydrogen into heavier elements. After the hydrogen is
   consumed, a star may become a red giant or evolve into
   other types of star not within the main sequence region.
   [PJC]